[PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
changed hvf_wfi() from blocking the vCPU thread with pselect() to
returning EXCP_HLT, intending QEMU's main event loop to handle the
idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
always returns false and the vCPU thread spins at 100% CPU per core
while the guest is idle.

Fix this by:

1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
   halt_cond in qemu_process_cpu_events().

2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
   hv_vcpu_run(), which is not called while the CPU is halted. The
   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
   vtimer IRQ through the GIC and marks vtimer_masked, causing the
   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().

3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
   indicates a pending interrupt, and cancelling the WFI timer.

Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959
Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
---
 include/system/hvf_int.h |  1 +
 target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
index 2621164cb2..58fb865eba 100644
--- a/include/system/hvf_int.h
+++ b/include/system/hvf_int.h
@@ -48,6 +48,7 @@ struct AccelCPUState {
     hv_vcpu_exit_t *exit;
     bool vtimer_masked;
     bool guest_debug_enabled;
+    struct QEMUTimer *wfi_timer;
 #endif
 };
 
diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
index 5fc8f6bbbd..0e737d3583 100644
--- a/target/arm/hvf/hvf.c
+++ b/target/arm/hvf/hvf.c
@@ -28,6 +28,7 @@
 #include "hw/core/boards.h"
 #include "hw/core/irq.h"
 #include "qemu/main-loop.h"
+#include "qemu/timer.h"
 #include "system/cpus.h"
 #include "arm-powerctl.h"
 #include "target/arm/cpu.h"
@@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
 #define TMR_CTL_IMASK   (1 << 1)
 #define TMR_CTL_ISTATUS (1 << 2)
 
+static void hvf_wfi_timer_cb(void *opaque);
+
 static uint32_t chosen_ipa_bit_size;
 
 typedef struct HVFVTimer {
@@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
 {
     hv_return_t ret;
 
+    timer_free(cpu->accel->wfi_timer);
+    cpu->accel->wfi_timer = NULL;
+
     ret = hv_vcpu_destroy(cpu->accel->fd);
     assert_hvf_ok(ret);
 }
@@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
     assert_hvf_ok(ret);
 
+    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
+                                          hvf_wfi_timer_cb, cpu);
+
     aarch64_add_sme_properties(OBJECT(cpu));
     return 0;
 }
@@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
+static void hvf_wfi_timer_cb(void *opaque)
+{
+    CPUState *cpu = opaque;
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+
+    /*
+     * vtimer expired while the CPU was halted for WFI.
+     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
+     * interrupt and mark as masked so hvf_sync_vtimer() will
+     * check and unmask when the guest handles it.
+     *
+     * The interrupt delivery chain (GIC -> cpu_interrupt ->
+     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
+     */
+    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
+    cpu->accel->vtimer_masked = true;
+}
+
 static int hvf_wfi(CPUState *cpu)
 {
+    uint64_t ctl, cval;
+    hv_return_t r;
+
     if (cpu_has_work(cpu)) {
         /*
          * Don't bother to go into our "low power state" if
@@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu)
         return 0;
     }
 
+    /*
+     * Set up a host-side timer to wake us when the vtimer expires.
+     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
+     * hv_vcpu_run(), which we won't call while halted.
+     */
+    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
+    assert_hvf_ok(r);
+
+    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
+        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
+                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
+        assert_hvf_ok(r);
+
+        uint64_t now = hvf_vtimer_val_raw();
+        if (cval <= now) {
+            /* Timer already expired, don't halt */
+            return 0;
+        }
+
+        uint64_t delta_ticks = cval - now;
+        mach_timebase_info_data_t timebase;
+        mach_timebase_info(&timebase);
+        int64_t delta_ns = delta_ticks * timebase.numer / timebase.denom;
+        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
+
+        timer_mod(cpu->accel->wfi_timer, deadline);
+    }
+
+    cpu->halted = 1;
     return EXCP_HLT;
 }
 
@@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
     hv_return_t r;
 
     if (cpu->halted) {
-        return EXCP_HLT;
+        if (!cpu_has_work(cpu)) {
+            return EXCP_HLT;
+        }
+        cpu->halted = 0;
+        timer_del(cpu->accel->wfi_timer);
     }
 
     flush_cpu_state(cpu);
-- 
2.50.1 (Apple Git-155)

Re: [PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 10. Apr 2026, at 06:47, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
> 
> Fix this by:
> 
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>   halt_cond in qemu_process_cpu_events().
> 
> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>   hv_vcpu_run(), which is not called while the CPU is halted. The
>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
> 
> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>   indicates a pending interrupt, and cancelling the WFI timer.
> 
> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959
> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>

Hi,

Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
> ---
> include/system/hvf_int.h |  1 +
> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
> index 2621164cb2..58fb865eba 100644
> --- a/include/system/hvf_int.h
> +++ b/include/system/hvf_int.h
> @@ -48,6 +48,7 @@ struct AccelCPUState {
>     hv_vcpu_exit_t *exit;
>     bool vtimer_masked;
>     bool guest_debug_enabled;
> +    struct QEMUTimer *wfi_timer;
> #endif
> };
> 
> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> index 5fc8f6bbbd..0e737d3583 100644
> --- a/target/arm/hvf/hvf.c
> +++ b/target/arm/hvf/hvf.c
> @@ -28,6 +28,7 @@
> #include "hw/core/boards.h"
> #include "hw/core/irq.h"
> #include "qemu/main-loop.h"
> +#include "qemu/timer.h"
> #include "system/cpus.h"
> #include "arm-powerctl.h"
> #include "target/arm/cpu.h"
> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
> #define TMR_CTL_IMASK   (1 << 1)
> #define TMR_CTL_ISTATUS (1 << 2)
> 
> +static void hvf_wfi_timer_cb(void *opaque);
> +
> static uint32_t chosen_ipa_bit_size;
> 
> typedef struct HVFVTimer {
> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
> {
>     hv_return_t ret;
> 
> +    timer_free(cpu->accel->wfi_timer);
> +    cpu->accel->wfi_timer = NULL;
> +
>     ret = hv_vcpu_destroy(cpu->accel->fd);
>     assert_hvf_ok(ret);
> }
> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>     assert_hvf_ok(ret);
> 
> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
> +                                          hvf_wfi_timer_cb, cpu);
> +
>     aarch64_add_sme_properties(OBJECT(cpu));
>     return 0;
> }
> @@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void)
>     return mach_absolute_time() - hvf_state->vtimer_offset;
> }
> 
> +static void hvf_wfi_timer_cb(void *opaque)
> +{
> +    CPUState *cpu = opaque;
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +
> +    /*
> +     * vtimer expired while the CPU was halted for WFI.
> +     * 	
> +     * interrupt and mark as masked so hvf_sync_vtimer() will
> +     * check and unmask when the guest handles it.
> +     *
> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
> +     */
> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
> +    cpu->accel->vtimer_masked = true;
> +}
> +
> static int hvf_wfi(CPUState *cpu)
> {
> +    uint64_t ctl, cval;
> +    hv_return_t r;
> +
>     if (cpu_has_work(cpu)) {
>         /*
>          * Don't bother to go into our "low power state" if
> @@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu)
>         return 0;
>     }
> 
> +    /*
> +     * Set up a host-side timer to wake us when the vtimer expires.
> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
> +     * hv_vcpu_run(), which we won't call while halted.
> +     */
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
> +    assert_hvf_ok(r);
> +
> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
> +        assert_hvf_ok(r);
> +
> +        uint64_t now = hvf_vtimer_val_raw();
> +        if (cval <= now) {
> +            /* Timer already expired, don't halt */
> +            return 0;
> +        }
> +
> +        uint64_t delta_ticks = cval - now;

We have CNTFRQ_EL0 in cpu->gt_cntfrq_hz but this works too
as XNU today tries to take care that host and VM run with
the same CNTFRQ_EL0

> +        mach_timebase_info_data_t timebase;
> +        mach_timebase_info(&timebase);
> +        int64_t delta_ns = delta_ticks * timebase.numer / timebase.denom;
> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
> +
> +        timer_mod(cpu->accel->wfi_timer, deadline);
> +    }
> +
> +    cpu->halted = 1;
>     return EXCP_HLT;
> }
> 
> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>     hv_return_t r;
> 
>     if (cpu->halted) {
> -        return EXCP_HLT;
> +        if (!cpu_has_work(cpu)) {
> +            return EXCP_HLT;
> +        }
> +        cpu->halted = 0;
> +        timer_del(cpu->accel->wfi_timer);
>     }
> 
>     flush_cpu_state(cpu);
> -- 
> 2.50.1 (Apple Git-155)
> 
> 

Re: [PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Thu Apr 9, 2026 at 10:06 PM PDT, Mohamed Mediouni wrote:
>
>
>> On 10. Apr 2026, at 06:47, Scott J. Goldman <scottjgo@gmail.com> wrote:
>> 
>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>> always returns false and the vCPU thread spins at 100% CPU per core
>> while the guest is idle.
>> 
>> Fix this by:
>> 
>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>   halt_cond in qemu_process_cpu_events().
>> 
>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>   hv_vcpu_run(), which is not called while the CPU is halted. The
>>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>> 
>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>   indicates a pending interrupt, and cancelling the WFI timer.
>> 
>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959
>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>
> Hi,
>
> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>> ---
>> include/system/hvf_int.h |  1 +
>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 65 insertions(+), 1 deletion(-)
>> 
>> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
>> index 2621164cb2..58fb865eba 100644
>> --- a/include/system/hvf_int.h
>> +++ b/include/system/hvf_int.h
>> @@ -48,6 +48,7 @@ struct AccelCPUState {
>>     hv_vcpu_exit_t *exit;
>>     bool vtimer_masked;
>>     bool guest_debug_enabled;
>> +    struct QEMUTimer *wfi_timer;
>> #endif
>> };
>> 
>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
>> index 5fc8f6bbbd..0e737d3583 100644
>> --- a/target/arm/hvf/hvf.c
>> +++ b/target/arm/hvf/hvf.c
>> @@ -28,6 +28,7 @@
>> #include "hw/core/boards.h"
>> #include "hw/core/irq.h"
>> #include "qemu/main-loop.h"
>> +#include "qemu/timer.h"
>> #include "system/cpus.h"
>> #include "arm-powerctl.h"
>> #include "target/arm/cpu.h"
>> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
>> #define TMR_CTL_IMASK   (1 << 1)
>> #define TMR_CTL_ISTATUS (1 << 2)
>> 
>> +static void hvf_wfi_timer_cb(void *opaque);
>> +
>> static uint32_t chosen_ipa_bit_size;
>> 
>> typedef struct HVFVTimer {
>> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
>> {
>>     hv_return_t ret;
>> 
>> +    timer_free(cpu->accel->wfi_timer);
>> +    cpu->accel->wfi_timer = NULL;
>> +
>>     ret = hv_vcpu_destroy(cpu->accel->fd);
>>     assert_hvf_ok(ret);
>> }
>> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>>                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>>     assert_hvf_ok(ret);
>> 
>> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
>> +                                          hvf_wfi_timer_cb, cpu);
>> +
>>     aarch64_add_sme_properties(OBJECT(cpu));
>>     return 0;
>> }
>> @@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void)
>>     return mach_absolute_time() - hvf_state->vtimer_offset;
>> }
>> 
>> +static void hvf_wfi_timer_cb(void *opaque)
>> +{
>> +    CPUState *cpu = opaque;
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +
>> +    /*
>> +     * vtimer expired while the CPU was halted for WFI.
>> +     * 	
>> +     * interrupt and mark as masked so hvf_sync_vtimer() will
>> +     * check and unmask when the guest handles it.
>> +     *
>> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
>> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
>> +     */
>> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
>> +    cpu->accel->vtimer_masked = true;
>> +}
>> +
>> static int hvf_wfi(CPUState *cpu)
>> {
>> +    uint64_t ctl, cval;
>> +    hv_return_t r;
>> +
>>     if (cpu_has_work(cpu)) {
>>         /*
>>          * Don't bother to go into our "low power state" if
>> @@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu)
>>         return 0;
>>     }
>> 
>> +    /*
>> +     * Set up a host-side timer to wake us when the vtimer expires.
>> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>> +     * hv_vcpu_run(), which we won't call while halted.
>> +     */
>> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
>> +    assert_hvf_ok(r);
>> +
>> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
>> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
>> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
>> +        assert_hvf_ok(r);
>> +
>> +        uint64_t now = hvf_vtimer_val_raw();
>> +        if (cval <= now) {
>> +            /* Timer already expired, don't halt */
>> +            return 0;
>> +        }
>> +
>> +        uint64_t delta_ticks = cval - now;
>
> We have CNTFRQ_EL0 in cpu->gt_cntfrq_hz but this works too
> as XNU today tries to take care that host and VM run with
> the same CNTFRQ_EL0
>

Ah, I can post a follow-up.

>> +        mach_timebase_info_data_t timebase;
>> +        mach_timebase_info(&timebase);
>> +        int64_t delta_ns = delta_ticks * timebase.numer / timebase.denom;
>> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
>> +
>> +        timer_mod(cpu->accel->wfi_timer, deadline);
>> +    }
>> +
>> +    cpu->halted = 1;
>>     return EXCP_HLT;
>> }
>> 
>> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>>     hv_return_t r;
>> 
>>     if (cpu->halted) {
>> -        return EXCP_HLT;
>> +        if (!cpu_has_work(cpu)) {
>> +            return EXCP_HLT;
>> +        }
>> +        cpu->halted = 0;
>> +        timer_del(cpu->accel->wfi_timer);
>>     }
>> 
>>     flush_cpu_state(cpu);
>> -- 
>> 2.50.1 (Apple Git-155)
>> 
>> 

Thanks,
-sjg

[PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
changed hvf_wfi() from blocking the vCPU thread with pselect() to
returning EXCP_HLT, intending QEMU's main event loop to handle the
idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
always returns false and the vCPU thread spins at 100% CPU per core
while the guest is idle.

Fix this by:

1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
   halt_cond in qemu_process_cpu_events().

2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
   hv_vcpu_run(), which is not called while the CPU is halted. The
   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
   vtimer IRQ through the GIC and marks vtimer_masked, causing the
   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().

3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
   indicates a pending interrupt, and cancelling the WFI timer.

Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
---
 include/system/hvf_int.h |  1 +
 target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
index 2621164cb2..58fb865eba 100644
--- a/include/system/hvf_int.h
+++ b/include/system/hvf_int.h
@@ -48,6 +48,7 @@ struct AccelCPUState {
     hv_vcpu_exit_t *exit;
     bool vtimer_masked;
     bool guest_debug_enabled;
+    struct QEMUTimer *wfi_timer;
 #endif
 };
 
diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
index 5fc8f6bbbd..f5d7221845 100644
--- a/target/arm/hvf/hvf.c
+++ b/target/arm/hvf/hvf.c
@@ -28,6 +28,7 @@
 #include "hw/core/boards.h"
 #include "hw/core/irq.h"
 #include "qemu/main-loop.h"
+#include "qemu/timer.h"
 #include "system/cpus.h"
 #include "arm-powerctl.h"
 #include "target/arm/cpu.h"
@@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
 #define TMR_CTL_IMASK   (1 << 1)
 #define TMR_CTL_ISTATUS (1 << 2)
 
+static void hvf_wfi_timer_cb(void *opaque);
+
 static uint32_t chosen_ipa_bit_size;
 
 typedef struct HVFVTimer {
@@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
 {
     hv_return_t ret;
 
+    timer_free(cpu->accel->wfi_timer);
+    cpu->accel->wfi_timer = NULL;
+
     ret = hv_vcpu_destroy(cpu->accel->fd);
     assert_hvf_ok(ret);
 }
@@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
     assert_hvf_ok(ret);
 
+    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
+                                          hvf_wfi_timer_cb, cpu);
+
     aarch64_add_sme_properties(OBJECT(cpu));
     return 0;
 }
@@ -2027,8 +2036,30 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
+static void hvf_wfi_timer_cb(void *opaque)
+{
+    CPUState *cpu = opaque;
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+
+    /*
+     * vtimer expired while the CPU was halted for WFI.
+     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
+     * interrupt and mark as masked so hvf_sync_vtimer() will
+     * check and unmask when the guest handles it.
+     *
+     * The interrupt delivery chain (GIC -> cpu_interrupt ->
+     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
+     */
+    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
+    cpu->accel->vtimer_masked = true;
+}
+
 static int hvf_wfi(CPUState *cpu)
 {
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+    uint64_t ctl, cval;
+    hv_return_t r;
+
     if (cpu_has_work(cpu)) {
         /*
          * Don't bother to go into our "low power state" if
@@ -2037,6 +2068,34 @@ static int hvf_wfi(CPUState *cpu)
         return 0;
     }
 
+    /*
+     * Set up a host-side timer to wake us when the vtimer expires.
+     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
+     * hv_vcpu_run(), which we won't call while halted.
+     */
+    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
+    assert_hvf_ok(r);
+
+    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
+        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
+                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
+        assert_hvf_ok(r);
+
+        uint64_t now = hvf_vtimer_val_raw();
+        if (cval <= now) {
+            /* Timer already expired, don't halt */
+            return 0;
+        }
+
+        uint64_t delta_ticks = cval - now;
+        int64_t delta_ns = delta_ticks * NANOSECONDS_PER_SECOND
+                           / arm_cpu->gt_cntfrq_hz;
+        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
+
+        timer_mod(cpu->accel->wfi_timer, deadline);
+    }
+
+    cpu->halted = 1;
     return EXCP_HLT;
 }
 
@@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
     hv_return_t r;
 
     if (cpu->halted) {
-        return EXCP_HLT;
+        if (!cpu_has_work(cpu)) {
+            return EXCP_HLT;
+        }
+        cpu->halted = 0;
+        timer_del(cpu->accel->wfi_timer);
     }
 
     flush_cpu_state(cpu);
-- 
2.50.1 (Apple Git-155)

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
> 
> Fix this by:
> 
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>   halt_cond in qemu_process_cpu_events().
> 
> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>   hv_vcpu_run(), which is not called while the CPU is halted. The
>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
> 
> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>   indicates a pending interrupt, and cancelling the WFI timer.
> 
> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>

Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
> ---
> include/system/hvf_int.h |  1 +
> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
> index 2621164cb2..58fb865eba 100644
> --- a/include/system/hvf_int.h
> +++ b/include/system/hvf_int.h
> @@ -48,6 +48,7 @@ struct AccelCPUState {
>     hv_vcpu_exit_t *exit;
>     bool vtimer_masked;
>     bool guest_debug_enabled;
> +    struct QEMUTimer *wfi_timer;
> #endif
> };
> 
> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> index 5fc8f6bbbd..f5d7221845 100644
> --- a/target/arm/hvf/hvf.c
> +++ b/target/arm/hvf/hvf.c
> @@ -28,6 +28,7 @@
> #include "hw/core/boards.h"
> #include "hw/core/irq.h"
> #include "qemu/main-loop.h"
> +#include "qemu/timer.h"
> #include "system/cpus.h"
> #include "arm-powerctl.h"
> #include "target/arm/cpu.h"
> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
> #define TMR_CTL_IMASK   (1 << 1)
> #define TMR_CTL_ISTATUS (1 << 2)
> 
> +static void hvf_wfi_timer_cb(void *opaque);
> +
> static uint32_t chosen_ipa_bit_size;
> 
> typedef struct HVFVTimer {
> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
> {
>     hv_return_t ret;
> 
> +    timer_free(cpu->accel->wfi_timer);
> +    cpu->accel->wfi_timer = NULL;
> +
>     ret = hv_vcpu_destroy(cpu->accel->fd);
>     assert_hvf_ok(ret);
> }
> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>     assert_hvf_ok(ret);
> 
> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
> +                                          hvf_wfi_timer_cb, cpu);
> +
>     aarch64_add_sme_properties(OBJECT(cpu));
>     return 0;
> }
> @@ -2027,8 +2036,30 @@ static uint64_t hvf_vtimer_val_raw(void)
>     return mach_absolute_time() - hvf_state->vtimer_offset;
> }
> 
> +static void hvf_wfi_timer_cb(void *opaque)
> +{
> +    CPUState *cpu = opaque;
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +
> +    /*
> +     * vtimer expired while the CPU was halted for WFI.
> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
> +     * interrupt and mark as masked so hvf_sync_vtimer() will
> +     * check and unmask when the guest handles it.
> +     *
> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
> +     */
> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
> +    cpu->accel->vtimer_masked = true;
> +}
> +
> static int hvf_wfi(CPUState *cpu)
> {
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +    uint64_t ctl, cval;
> +    hv_return_t r;
> +
>     if (cpu_has_work(cpu)) {
>         /*
>          * Don't bother to go into our "low power state" if
> @@ -2037,6 +2068,34 @@ static int hvf_wfi(CPUState *cpu)
>         return 0;
>     }
> 
> +    /*
> +     * Set up a host-side timer to wake us when the vtimer expires.
> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
> +     * hv_vcpu_run(), which we won't call while halted.
> +     */
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
> +    assert_hvf_ok(r);
> +
> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
> +        assert_hvf_ok(r);
> +
> +        uint64_t now = hvf_vtimer_val_raw();
> +        if (cval <= now) {
> +            /* Timer already expired, don't halt */
> +            return 0;
> +        }
> +
> +        uint64_t delta_ticks = cval - now;
> +        int64_t delta_ns = delta_ticks * NANOSECONDS_PER_SECOND
> +                           / arm_cpu->gt_cntfrq_hz;
> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
> +
> +        timer_mod(cpu->accel->wfi_timer, deadline);
> +    }
> +
> +    cpu->halted = 1;
>     return EXCP_HLT;
> }
> 
> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>     hv_return_t r;
> 
>     if (cpu->halted) {
> -        return EXCP_HLT;
> +        if (!cpu_has_work(cpu)) {
> +            return EXCP_HLT;
> +        }
> +        cpu->halted = 0;
> +        timer_del(cpu->accel->wfi_timer);
>     }
> 
>     flush_cpu_state(cpu);
> -- 
> 2.50.1 (Apple Git-155)
> 
> 

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

Philippe-- apologies for the nag here but this is a fix for a unfortunate
regression. Any chance you could take a look? Thanks!

On Thu Apr 9, 2026 at 11:18 PM PDT, Mohamed Mediouni wrote:
>
>
>> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>> 
>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>> always returns false and the vCPU thread spins at 100% CPU per core
>> while the guest is idle.
>> 
>> Fix this by:
>> 
>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>   halt_cond in qemu_process_cpu_events().
>> 
>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>   hv_vcpu_run(), which is not called while the CPU is halted. The
>>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>> 
>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>   indicates a pending interrupt, and cancelling the WFI timer.
>> 
>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>
> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>> ---
>> include/system/hvf_int.h |  1 +
>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 65 insertions(+), 1 deletion(-)
>> 
>> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
>> index 2621164cb2..58fb865eba 100644
>> --- a/include/system/hvf_int.h
>> +++ b/include/system/hvf_int.h
>> @@ -48,6 +48,7 @@ struct AccelCPUState {
>>     hv_vcpu_exit_t *exit;
>>     bool vtimer_masked;
>>     bool guest_debug_enabled;
>> +    struct QEMUTimer *wfi_timer;
>> #endif
>> };
>> 
>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
>> index 5fc8f6bbbd..f5d7221845 100644
>> --- a/target/arm/hvf/hvf.c
>> +++ b/target/arm/hvf/hvf.c
>> @@ -28,6 +28,7 @@
>> #include "hw/core/boards.h"
>> #include "hw/core/irq.h"
>> #include "qemu/main-loop.h"
>> +#include "qemu/timer.h"
>> #include "system/cpus.h"
>> #include "arm-powerctl.h"
>> #include "target/arm/cpu.h"
>> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
>> #define TMR_CTL_IMASK   (1 << 1)
>> #define TMR_CTL_ISTATUS (1 << 2)
>> 
>> +static void hvf_wfi_timer_cb(void *opaque);
>> +
>> static uint32_t chosen_ipa_bit_size;
>> 
>> typedef struct HVFVTimer {
>> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
>> {
>>     hv_return_t ret;
>> 
>> +    timer_free(cpu->accel->wfi_timer);
>> +    cpu->accel->wfi_timer = NULL;
>> +
>>     ret = hv_vcpu_destroy(cpu->accel->fd);
>>     assert_hvf_ok(ret);
>> }
>> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>>                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>>     assert_hvf_ok(ret);
>> 
>> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,
>> +                                          hvf_wfi_timer_cb, cpu);
>> +
>>     aarch64_add_sme_properties(OBJECT(cpu));
>>     return 0;
>> }
>> @@ -2027,8 +2036,30 @@ static uint64_t hvf_vtimer_val_raw(void)
>>     return mach_absolute_time() - hvf_state->vtimer_offset;
>> }
>> 
>> +static void hvf_wfi_timer_cb(void *opaque)
>> +{
>> +    CPUState *cpu = opaque;
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +
>> +    /*
>> +     * vtimer expired while the CPU was halted for WFI.
>> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
>> +     * interrupt and mark as masked so hvf_sync_vtimer() will
>> +     * check and unmask when the guest handles it.
>> +     *
>> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
>> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
>> +     */
>> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
>> +    cpu->accel->vtimer_masked = true;
>> +}
>> +
>> static int hvf_wfi(CPUState *cpu)
>> {
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +    uint64_t ctl, cval;
>> +    hv_return_t r;
>> +
>>     if (cpu_has_work(cpu)) {
>>         /*
>>          * Don't bother to go into our "low power state" if
>> @@ -2037,6 +2068,34 @@ static int hvf_wfi(CPUState *cpu)
>>         return 0;
>>     }
>> 
>> +    /*
>> +     * Set up a host-side timer to wake us when the vtimer expires.
>> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>> +     * hv_vcpu_run(), which we won't call while halted.
>> +     */
>> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
>> +    assert_hvf_ok(r);
>> +
>> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
>> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
>> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
>> +        assert_hvf_ok(r);
>> +
>> +        uint64_t now = hvf_vtimer_val_raw();
>> +        if (cval <= now) {
>> +            /* Timer already expired, don't halt */
>> +            return 0;
>> +        }
>> +
>> +        uint64_t delta_ticks = cval - now;
>> +        int64_t delta_ns = delta_ticks * NANOSECONDS_PER_SECOND
>> +                           / arm_cpu->gt_cntfrq_hz;
>> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
>> +
>> +        timer_mod(cpu->accel->wfi_timer, deadline);
>> +    }
>> +
>> +    cpu->halted = 1;
>>     return EXCP_HLT;
>> }
>> 
>> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>>     hv_return_t r;
>> 
>>     if (cpu->halted) {
>> -        return EXCP_HLT;
>> +        if (!cpu_has_work(cpu)) {
>> +            return EXCP_HLT;
>> +        }
>> +        cpu->halted = 0;
>> +        timer_del(cpu->accel->wfi_timer);
>>     }
>> 
>>     flush_cpu_state(cpu);
>> -- 
>> 2.50.1 (Apple Git-155)
>> 
>> 

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Philippe Mathieu-Daudé]

Hi Scott,

On 16/4/26 23:20, Scott J. Goldman wrote:
> Philippe-- apologies for the nag here but this is a fix for a unfortunate
> regression. Any chance you could take a look? Thanks!
> 
> On Thu Apr 9, 2026 at 11:18 PM PDT, Mohamed Mediouni wrote:
>>
>>
>>> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>>
>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>> always returns false and the vCPU thread spins at 100% CPU per core
>>> while the guest is idle.
>>>
>>> Fix this by:
>>>
>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>>    halt_cond in qemu_process_cpu_events().
>>>
>>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>>    hv_vcpu_run(), which is not called while the CPU is halted. The
>>>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>>
>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>>    indicates a pending interrupt, and cancelling the WFI timer.
>>>
>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>>
>> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>>> ---
>>> include/system/hvf_int.h |  1 +
>>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>>> 2 files changed, 65 insertions(+), 1 deletion(-)

I haven't looked at your patch yet. While looking at commit
b5f8f77271 I noticed the previous a14afa985e3 ("accel/hvf: Skip
WFI if CPU has work to do") is different in my local tree, where
I have:

-- >8 --
diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
index 5fc8f6bbbd9..1a76c9cf402 100644
--- a/target/arm/hvf/hvf.c
+++ b/target/arm/hvf/hvf.c
@@ -2037,6 +2037,11 @@ static int hvf_wfi(CPUState *cpu)
          return 0;
      }

+    if (cpu_test_interrupt(cpu, CPU_INTERRUPT_HARD | CPU_INTERRUPT_FIQ)) {
+        /* Interrupt pending, no need to wait */
+        return EXCP_INTERRUPT;
+    }
+
      return EXCP_HLT;
  }

---

Does that help? Now I'm really confused.

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Fri Apr 17, 2026 at 2:57 AM PDT, Philippe Mathieu-Daudé wrote:
> Hi Scott,
>
> On 16/4/26 23:20, Scott J. Goldman wrote:
>> Philippe-- apologies for the nag here but this is a fix for a unfortunate
>> regression. Any chance you could take a look? Thanks!
>> 
>> On Thu Apr 9, 2026 at 11:18 PM PDT, Mohamed Mediouni wrote:
>>>
>>>
>>>> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>>>
>>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>>> always returns false and the vCPU thread spins at 100% CPU per core
>>>> while the guest is idle.
>>>>
>>>> Fix this by:
>>>>
>>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>>>    halt_cond in qemu_process_cpu_events().
>>>>
>>>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>>>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>>>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>>>    hv_vcpu_run(), which is not called while the CPU is halted. The
>>>>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>>>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>>>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>>>
>>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>>>    indicates a pending interrupt, and cancelling the WFI timer.
>>>>
>>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>>>
>>> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>>>> ---
>>>> include/system/hvf_int.h |  1 +
>>>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>>>> 2 files changed, 65 insertions(+), 1 deletion(-)
>
> I haven't looked at your patch yet. While looking at commit
> b5f8f77271 I noticed the previous a14afa985e3 ("accel/hvf: Skip
> WFI if CPU has work to do") is different in my local tree, where
> I have:
>
> -- >8 --
> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> index 5fc8f6bbbd9..1a76c9cf402 100644
> --- a/target/arm/hvf/hvf.c
> +++ b/target/arm/hvf/hvf.c
> @@ -2037,6 +2037,11 @@ static int hvf_wfi(CPUState *cpu)
>           return 0;
>       }
>
> +    if (cpu_test_interrupt(cpu, CPU_INTERRUPT_HARD | CPU_INTERRUPT_FIQ)) {
> +        /* Interrupt pending, no need to wait */
> +        return EXCP_INTERRUPT;
> +    }
> +
>       return EXCP_HLT;
>   }
>
> ---
>
> Does that help? Now I'm really confused.


Hi Phillipe-- Appreciate your help here. The good news is the bug is
very easy to reproduce (start a linux VM with 1 cpu, watch the vCPU on
the host will always spin 100%). The bad news is I double checked my
bisect and you can see before b5f8f77271 there is no problem, and after
the commit is introduced, the problem appears. I also applied the
additional cpu_test_interrupt() patch you suggested and it did not
change the behavior.

thanks
-sjg

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Fri Apr 17, 2026 at 1:30 PM PDT, Scott J. Goldman wrote:
> On Fri Apr 17, 2026 at 2:57 AM PDT, Philippe Mathieu-Daudé wrote:
>> Hi Scott,
>>
>> On 16/4/26 23:20, Scott J. Goldman wrote:
>>> Philippe-- apologies for the nag here but this is a fix for a unfortunate
>>> regression. Any chance you could take a look? Thanks!
>>> 
>>> On Thu Apr 9, 2026 at 11:18 PM PDT, Mohamed Mediouni wrote:
>>>>
>>>>
>>>>> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>>>>
>>>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>>>> always returns false and the vCPU thread spins at 100% CPU per core
>>>>> while the guest is idle.
>>>>>
>>>>> Fix this by:
>>>>>
>>>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>>>>    halt_cond in qemu_process_cpu_events().
>>>>>
>>>>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>>>>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>>>>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>>>>    hv_vcpu_run(), which is not called while the CPU is halted. The
>>>>>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>>>>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>>>>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>>>>
>>>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>>>>    indicates a pending interrupt, and cancelling the WFI timer.
>>>>>
>>>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>>>>
>>>> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>>>>> ---
>>>>> include/system/hvf_int.h |  1 +
>>>>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>>>>> 2 files changed, 65 insertions(+), 1 deletion(-)
>>
>> I haven't looked at your patch yet. While looking at commit
>> b5f8f77271 I noticed the previous a14afa985e3 ("accel/hvf: Skip
>> WFI if CPU has work to do") is different in my local tree, where
>> I have:
>>
>> -- >8 --
>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
>> index 5fc8f6bbbd9..1a76c9cf402 100644
>> --- a/target/arm/hvf/hvf.c
>> +++ b/target/arm/hvf/hvf.c
>> @@ -2037,6 +2037,11 @@ static int hvf_wfi(CPUState *cpu)
>>           return 0;
>>       }
>>
>> +    if (cpu_test_interrupt(cpu, CPU_INTERRUPT_HARD | CPU_INTERRUPT_FIQ)) {
>> +        /* Interrupt pending, no need to wait */
>> +        return EXCP_INTERRUPT;
>> +    }
>> +
>>       return EXCP_HLT;
>>   }
>>
>> ---
>>
>> Does that help? Now I'm really confused.
>
>
> Hi Phillipe-- Appreciate your help here. The good news is the bug is
> very easy to reproduce (start a linux VM with 1 cpu, watch the vCPU on
> the host will always spin 100%). The bad news is I double checked my
> bisect and you can see before b5f8f77271 there is no problem, and after
> the commit is introduced, the problem appears. I also applied the
> additional cpu_test_interrupt() patch you suggested and it did not
> change the behavior.
>
> thanks
> -sjg

Hi Philippe- Apologies again for bugging you, but it looks like v11 is
about to be released and it would be a bummer if this regression went
out with the stable version. Have you had a chance to either look at my
patch or check if there was some other simpler fix?

Also, I'm not a regular qemu contributor, so if there is some process
here, that I'm missing, please let me know.

thanks
-sjg

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Peter Maydell]

On Fri, 10 Apr 2026 at 06:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
>
> Fix this by:
>
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>    halt_cond in qemu_process_cpu_events().
>
> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>    hv_vcpu_run(), which is not called while the CPU is halted. The
>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().

A host side timer here seems a bit odd -- it will still
expire even if the user halts the whole VM. We only use
QEMU_CLOCK_HOST timers for things that are part of QEMU proper,
like handling timeouts in the migration networking code.

> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
> index 2621164cb2..58fb865eba 100644
> --- a/include/system/hvf_int.h
> +++ b/include/system/hvf_int.h
> @@ -48,6 +48,7 @@ struct AccelCPUState {
>      hv_vcpu_exit_t *exit;
>      bool vtimer_masked;
>      bool guest_debug_enabled;
> +    struct QEMUTimer *wfi_timer;
>  #endif
>  };

QEMUTimer objects need to be migrated. I think as it is now,
if you do a migration while one vCPU is in WFI we won't
migrate the timer state, and so on the destination it won't
be woken up when it should.

> +static void hvf_wfi_timer_cb(void *opaque)
> +{
> +    CPUState *cpu = opaque;
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +
> +    /*
> +     * vtimer expired while the CPU was halted for WFI.
> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
> +     * interrupt and mark as masked so hvf_sync_vtimer() will
> +     * check and unmask when the guest handles it.
> +     *
> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
> +     */
> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
> +    cpu->accel->vtimer_masked = true;

If the VM happens to be stopped by the user when the timer
fires, this will change the VM state, which I suspect is
not correct. Avoiding use of a QEMU_CLOCK_HOST timer would
fix this.

> +}
> +
>  static int hvf_wfi(CPUState *cpu)
>  {
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +    uint64_t ctl, cval;
> +    hv_return_t r;
> +
>      if (cpu_has_work(cpu)) {
>          /*
>           * Don't bother to go into our "low power state" if
> @@ -2037,6 +2068,34 @@ static int hvf_wfi(CPUState *cpu)
>          return 0;
>      }
>
> +    /*
> +     * Set up a host-side timer to wake us when the vtimer expires.
> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
> +     * hv_vcpu_run(), which we won't call while halted.
> +     */
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
> +    assert_hvf_ok(r);
> +
> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
> +        assert_hvf_ok(r);
> +
> +        uint64_t now = hvf_vtimer_val_raw();
> +        if (cval <= now) {
> +            /* Timer already expired, don't halt */
> +            return 0;
> +        }
> +
> +        uint64_t delta_ticks = cval - now;
> +        int64_t delta_ns = delta_ticks * NANOSECONDS_PER_SECOND
> +                           / arm_cpu->gt_cntfrq_hz;

As a general rule, tick-to-ns and vice-versa conversions should
use muldiv64() to avoid potential overflow in the intermediate value.

> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
> +
> +        timer_mod(cpu->accel->wfi_timer, deadline);
> +    }
> +
> +    cpu->halted = 1;
>      return EXCP_HLT;
>  }

thanks
-- PMM

Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Mon Apr 27, 2026 at 4:15 AM PDT, Peter Maydell wrote:
> On Fri, 10 Apr 2026 at 06:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>
>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>> always returns false and the vCPU thread spins at 100% CPU per core
>> while the guest is idle.
>>
>> Fix this by:
>>
>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>    halt_cond in qemu_process_cpu_events().
>>
>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>    hv_vcpu_run(), which is not called while the CPU is halted. The
>>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>
> A host side timer here seems a bit odd -- it will still
> expire even if the user halts the whole VM. We only use
> QEMU_CLOCK_HOST timers for things that are part of QEMU proper,
> like handling timeouts in the migration networking code.
>
>> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
>> index 2621164cb2..58fb865eba 100644
>> --- a/include/system/hvf_int.h
>> +++ b/include/system/hvf_int.h
>> @@ -48,6 +48,7 @@ struct AccelCPUState {
>>      hv_vcpu_exit_t *exit;
>>      bool vtimer_masked;
>>      bool guest_debug_enabled;
>> +    struct QEMUTimer *wfi_timer;
>>  #endif
>>  };
>
> QEMUTimer objects need to be migrated. I think as it is now,
> if you do a migration while one vCPU is in WFI we won't
> migrate the timer state, and so on the destination it won't
> be woken up when it should.

Hi Peter--

Thanks for the review here. I am going to post a follow-up patch based
on your comments, but I did want to just flag that migration is broken
in hvf/arm right now (segfaults in the dirty page tracking code). Of
course, I can post fixes for that as well.


>
>> +static void hvf_wfi_timer_cb(void *opaque)
>> +{
>> +    CPUState *cpu = opaque;
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +
>> +    /*
>> +     * vtimer expired while the CPU was halted for WFI.
>> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
>> +     * interrupt and mark as masked so hvf_sync_vtimer() will
>> +     * check and unmask when the guest handles it.
>> +     *
>> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
>> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
>> +     */
>> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
>> +    cpu->accel->vtimer_masked = true;
>
> If the VM happens to be stopped by the user when the timer
> fires, this will change the VM state, which I suspect is
> not correct. Avoiding use of a QEMU_CLOCK_HOST timer would
> fix this.
>
>> +}
>> +
>>  static int hvf_wfi(CPUState *cpu)
>>  {
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +    uint64_t ctl, cval;
>> +    hv_return_t r;
>> +
>>      if (cpu_has_work(cpu)) {
>>          /*
>>           * Don't bother to go into our "low power state" if
>> @@ -2037,6 +2068,34 @@ static int hvf_wfi(CPUState *cpu)
>>          return 0;
>>      }
>>
>> +    /*
>> +     * Set up a host-side timer to wake us when the vtimer expires.
>> +     * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>> +     * hv_vcpu_run(), which we won't call while halted.
>> +     */
>> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
>> +    assert_hvf_ok(r);
>> +
>> +    if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {
>> +        r = hv_vcpu_get_sys_reg(cpu->accel->fd,
>> +                                HV_SYS_REG_CNTV_CVAL_EL0, &cval);
>> +        assert_hvf_ok(r);
>> +
>> +        uint64_t now = hvf_vtimer_val_raw();
>> +        if (cval <= now) {
>> +            /* Timer already expired, don't halt */
>> +            return 0;
>> +        }
>> +
>> +        uint64_t delta_ticks = cval - now;
>> +        int64_t delta_ns = delta_ticks * NANOSECONDS_PER_SECOND
>> +                           / arm_cpu->gt_cntfrq_hz;
>
> As a general rule, tick-to-ns and vice-versa conversions should
> use muldiv64() to avoid potential overflow in the intermediate value.
>
>> +        int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;
>> +
>> +        timer_mod(cpu->accel->wfi_timer, deadline);
>> +    }
>> +
>> +    cpu->halted = 1;
>>      return EXCP_HLT;
>>  }
>
> thanks
> -- PMM

[PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
changed hvf_wfi() from blocking the vCPU thread with pselect() to
returning EXCP_HLT, intending QEMU's main event loop to handle the
idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
always returns false and the vCPU thread spins at 100% CPU per core
while the guest is idle.

Fix this by:

1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
   halt_cond in qemu_process_cpu_events().

2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
   hv_vcpu_run(), which is not called while the CPU is halted. The
   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
   vtimer IRQ through the GIC and marks vtimer_masked, causing the
   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().

3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
   indicates a pending interrupt, and cancelling the WFI timer.

4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
   transition for any halted vCPU, since the QEMUTimer is per-instance
   state and is not migrated. After cpu_synchronize_all_states() the
   migrated vtimer state is mirrored in env, so we can read CNTV_CTL
   and CNTV_CVAL from there. If the vtimer has already expired by the
   time the destination resumes, hvf_wfi_timer_cb() is invoked
   directly so the halted vCPU is woken up.

Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
---
Changes since v2:
 - Use QEMU_CLOCK_VIRTUAL instead of QEMU_CLOCK_HOST so the timer
   pauses with the VM and a halted vCPU isn't woken (or its IRQ
   raised) while the user has stopped the guest. (Peter)
 - Convert vtimer ticks to nanoseconds with muldiv64() to avoid
   intermediate overflow. (Peter)
 - Re-arm the WFI timer from hvf_vm_state_change() on the resume
   transition so a halted vCPU on the migration destination is
   woken when its vtimer expires (the QEMUTimer is per-instance
   state and isn't migrated). (Peter)
v2: https://lore.kernel.org/qemu-devel/20260410055045.63001-1-scottjgo@gmail.com/
v1: https://lore.kernel.org/qemu-devel/20260410044726.61853-1-scottjgo@gmail.com/

 include/system/hvf_int.h |   1 +
 target/arm/hvf/hvf.c     | 124 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 124 insertions(+), 1 deletion(-)

diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
index 2621164cb2..58fb865eba 100644
--- a/include/system/hvf_int.h
+++ b/include/system/hvf_int.h
@@ -48,6 +48,7 @@ struct AccelCPUState {
     hv_vcpu_exit_t *exit;
     bool vtimer_masked;
     bool guest_debug_enabled;
+    struct QEMUTimer *wfi_timer;
 #endif
 };
 
diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
index 678afe5c8e..a19d7a5e1f 100644
--- a/target/arm/hvf/hvf.c
+++ b/target/arm/hvf/hvf.c
@@ -28,6 +28,7 @@
 #include "hw/core/boards.h"
 #include "hw/core/irq.h"
 #include "qemu/main-loop.h"
+#include "qemu/timer.h"
 #include "system/cpus.h"
 #include "arm-powerctl.h"
 #include "target/arm/cpu.h"
@@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
 #define TMR_CTL_IMASK   (1 << 1)
 #define TMR_CTL_ISTATUS (1 << 2)
 
+static void hvf_wfi_timer_cb(void *opaque);
+
 static uint32_t chosen_ipa_bit_size;
 
 typedef struct HVFVTimer {
@@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
 {
     hv_return_t ret;
 
+    timer_free(cpu->accel->wfi_timer);
+    cpu->accel->wfi_timer = NULL;
+
     ret = hv_vcpu_destroy(cpu->accel->fd);
     assert_hvf_ok(ret);
 }
@@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
     assert_hvf_ok(ret);
 
+    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
+                                          hvf_wfi_timer_cb, cpu);
+
     aarch64_add_sme_properties(OBJECT(cpu));
     return 0;
 }
@@ -2027,8 +2036,67 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
+static void hvf_wfi_timer_cb(void *opaque)
+{
+    CPUState *cpu = opaque;
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+
+    /*
+     * vtimer expired while the CPU was halted for WFI.
+     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
+     * interrupt and mark as masked so hvf_sync_vtimer() will
+     * check and unmask when the guest handles it.
+     *
+     * The interrupt delivery chain (GIC -> cpu_interrupt ->
+     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
+     */
+    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
+    cpu->accel->vtimer_masked = true;
+}
+
+/*
+ * Arm a host-side QEMU_CLOCK_VIRTUAL timer to fire when the guest's
+ * vtimer (CNTV_CVAL_EL0) is scheduled to expire. HVF only delivers
+ * HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which we won't
+ * call while the vCPU is halted, so we need this to wake the vCPU.
+ *
+ * QEMU_CLOCK_VIRTUAL pauses while the VM is stopped, which keeps the
+ * timer in lockstep with the guest's view of vtime across pause/resume.
+ *
+ * Caller must supply the current CNTV_CTL_EL0 and CNTV_CVAL_EL0 values,
+ * since the appropriate source (HVF vs. env) depends on context.
+ *
+ * Returns 0 if the timer was armed (or if the vtimer is disabled/masked
+ * and the vCPU should still halt waiting on another event), or -1 if
+ * the vtimer has already expired.
+ */
+static int hvf_arm_wfi_timer(CPUState *cpu, uint64_t ctl, uint64_t cval)
+{
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+    uint64_t now;
+    int64_t delta_ns;
+
+    if (!(ctl & TMR_CTL_ENABLE) || (ctl & TMR_CTL_IMASK)) {
+        return 0;
+    }
+
+    now = hvf_vtimer_val_raw();
+    if (cval <= now) {
+        return -1;
+    }
+
+    delta_ns = muldiv64(cval - now, NANOSECONDS_PER_SECOND,
+                        arm_cpu->gt_cntfrq_hz);
+    timer_mod(cpu->accel->wfi_timer,
+              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + delta_ns);
+    return 0;
+}
+
 static int hvf_wfi(CPUState *cpu)
 {
+    uint64_t ctl, cval;
+    hv_return_t r;
+
     if (cpu_has_work(cpu)) {
         /*
          * Don't bother to go into our "low power state" if
@@ -2037,6 +2105,22 @@ static int hvf_wfi(CPUState *cpu)
         return 0;
     }
 
+    /*
+     * Read the vtimer state directly from HVF. We're on the vCPU thread,
+     * just exited from hv_vcpu_run(), so HVF holds the authoritative
+     * values and env may be stale.
+     */
+    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
+    assert_hvf_ok(r);
+    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CVAL_EL0, &cval);
+    assert_hvf_ok(r);
+
+    if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
+        /* vtimer already expired, don't halt */
+        return 0;
+    }
+
+    cpu->halted = 1;
     return EXCP_HLT;
 }
 
@@ -2332,7 +2416,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
     hv_return_t r;
 
     if (cpu->halted) {
-        return EXCP_HLT;
+        if (!cpu_has_work(cpu)) {
+            return EXCP_HLT;
+        }
+        cpu->halted = 0;
+        timer_del(cpu->accel->wfi_timer);
     }
 
     flush_cpu_state(cpu);
@@ -2376,11 +2464,45 @@ static const VMStateDescription vmstate_hvf_vtimer = {
 static void hvf_vm_state_change(void *opaque, bool running, RunState state)
 {
     HVFVTimer *s = opaque;
+    CPUState *cpu;
 
     if (running) {
         /* Update vtimer offset on all CPUs */
         hvf_state->vtimer_offset = mach_absolute_time() - s->vtimer_val;
         cpu_synchronize_all_states();
+
+        /*
+         * After migration restore (or any resume), the wfi_timer is not
+         * scheduled on this QEMU instance, so re-arm it for any halted
+         * vCPU with a pending vtimer. For a non-migration resume the
+         * QEMU_CLOCK_VIRTUAL timer was already scheduled; recomputing the
+         * deadline produces the same value and is a harmless no-op.
+         *
+         * cpu_synchronize_all_states() above ensures env mirrors the
+         * authoritative vtimer state (whether that came from HVF or from
+         * the migration stream), so we can safely read it here from the
+         * iothread.
+         */
+        CPU_FOREACH(cpu) {
+            ARMCPU *arm_cpu;
+            uint64_t ctl, cval;
+
+            if (!cpu->accel || !cpu->halted) {
+                continue;
+            }
+
+            arm_cpu = ARM_CPU(cpu);
+            ctl = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].ctl;
+            cval = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].cval;
+
+            if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
+                /*
+                 * vtimer already expired while we were paused; raise the
+                 * IRQ now so the halted vCPU wakes up.
+                 */
+                hvf_wfi_timer_cb(cpu);
+            }
+        }
     } else {
         /* Remember vtimer value on every pause */
         s->vtimer_val = hvf_vtimer_val_raw();
-- 
2.50.1 (Apple Git-155)

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Mon Apr 27, 2026 at 3:55 PM EDT, Scott J. Goldman wrote:
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
>
> Fix this by:
>
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>    halt_cond in qemu_process_cpu_events().
>
> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>    hv_vcpu_run(), which is not called while the CPU is halted. The
>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>
> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>    indicates a pending interrupt, and cancelling the WFI timer.
>
> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>    transition for any halted vCPU, since the QEMUTimer is per-instance
>    state and is not migrated. After cpu_synchronize_all_states() the
>    migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>    and CNTV_CVAL from there. If the vtimer has already expired by the
>    time the destination resumes, hvf_wfi_timer_cb() is invoked
>    directly so the halted vCPU is woken up.
>
> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
> ---
> Changes since v2:
>  - Use QEMU_CLOCK_VIRTUAL instead of QEMU_CLOCK_HOST so the timer
>    pauses with the VM and a halted vCPU isn't woken (or its IRQ
>    raised) while the user has stopped the guest. (Peter)
>  - Convert vtimer ticks to nanoseconds with muldiv64() to avoid
>    intermediate overflow. (Peter)
>  - Re-arm the WFI timer from hvf_vm_state_change() on the resume
>    transition so a halted vCPU on the migration destination is
>    woken when its vtimer expires (the QEMUTimer is per-instance
>    state and isn't migrated). (Peter)
> v2: https://lore.kernel.org/qemu-devel/20260410055045.63001-1-scottjgo@gmail.com/
> v1: https://lore.kernel.org/qemu-devel/20260410044726.61853-1-scottjgo@gmail.com/
>
>  include/system/hvf_int.h |   1 +
>  target/arm/hvf/hvf.c     | 124 ++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 124 insertions(+), 1 deletion(-)
>
> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
> index 2621164cb2..58fb865eba 100644
> --- a/include/system/hvf_int.h
> +++ b/include/system/hvf_int.h
> @@ -48,6 +48,7 @@ struct AccelCPUState {
>      hv_vcpu_exit_t *exit;
>      bool vtimer_masked;
>      bool guest_debug_enabled;
> +    struct QEMUTimer *wfi_timer;
>  #endif
>  };
>  
> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> index 678afe5c8e..a19d7a5e1f 100644
> --- a/target/arm/hvf/hvf.c
> +++ b/target/arm/hvf/hvf.c
> @@ -28,6 +28,7 @@
>  #include "hw/core/boards.h"
>  #include "hw/core/irq.h"
>  #include "qemu/main-loop.h"
> +#include "qemu/timer.h"
>  #include "system/cpus.h"
>  #include "arm-powerctl.h"
>  #include "target/arm/cpu.h"
> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
>  #define TMR_CTL_IMASK   (1 << 1)
>  #define TMR_CTL_ISTATUS (1 << 2)
>  
> +static void hvf_wfi_timer_cb(void *opaque);
> +
>  static uint32_t chosen_ipa_bit_size;
>  
>  typedef struct HVFVTimer {
> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
>  {
>      hv_return_t ret;
>  
> +    timer_free(cpu->accel->wfi_timer);
> +    cpu->accel->wfi_timer = NULL;
> +
>      ret = hv_vcpu_destroy(cpu->accel->fd);
>      assert_hvf_ok(ret);
>  }
> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>                                arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>      assert_hvf_ok(ret);
>  
> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
> +                                          hvf_wfi_timer_cb, cpu);
> +
>      aarch64_add_sme_properties(OBJECT(cpu));
>      return 0;
>  }
> @@ -2027,8 +2036,67 @@ static uint64_t hvf_vtimer_val_raw(void)
>      return mach_absolute_time() - hvf_state->vtimer_offset;
>  }
>  
> +static void hvf_wfi_timer_cb(void *opaque)
> +{
> +    CPUState *cpu = opaque;
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +
> +    /*
> +     * vtimer expired while the CPU was halted for WFI.
> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
> +     * interrupt and mark as masked so hvf_sync_vtimer() will
> +     * check and unmask when the guest handles it.
> +     *
> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
> +     */
> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
> +    cpu->accel->vtimer_masked = true;
> +}
> +
> +/*
> + * Arm a host-side QEMU_CLOCK_VIRTUAL timer to fire when the guest's
> + * vtimer (CNTV_CVAL_EL0) is scheduled to expire. HVF only delivers
> + * HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which we won't
> + * call while the vCPU is halted, so we need this to wake the vCPU.
> + *
> + * QEMU_CLOCK_VIRTUAL pauses while the VM is stopped, which keeps the
> + * timer in lockstep with the guest's view of vtime across pause/resume.
> + *
> + * Caller must supply the current CNTV_CTL_EL0 and CNTV_CVAL_EL0 values,
> + * since the appropriate source (HVF vs. env) depends on context.
> + *
> + * Returns 0 if the timer was armed (or if the vtimer is disabled/masked
> + * and the vCPU should still halt waiting on another event), or -1 if
> + * the vtimer has already expired.
> + */
> +static int hvf_arm_wfi_timer(CPUState *cpu, uint64_t ctl, uint64_t cval)
> +{
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +    uint64_t now;
> +    int64_t delta_ns;
> +
> +    if (!(ctl & TMR_CTL_ENABLE) || (ctl & TMR_CTL_IMASK)) {
> +        return 0;
> +    }
> +
> +    now = hvf_vtimer_val_raw();
> +    if (cval <= now) {
> +        return -1;
> +    }
> +
> +    delta_ns = muldiv64(cval - now, NANOSECONDS_PER_SECOND,
> +                        arm_cpu->gt_cntfrq_hz);
> +    timer_mod(cpu->accel->wfi_timer,
> +              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + delta_ns);
> +    return 0;
> +}
> +
>  static int hvf_wfi(CPUState *cpu)
>  {
> +    uint64_t ctl, cval;
> +    hv_return_t r;
> +
>      if (cpu_has_work(cpu)) {
>          /*
>           * Don't bother to go into our "low power state" if
> @@ -2037,6 +2105,22 @@ static int hvf_wfi(CPUState *cpu)
>          return 0;
>      }
>  
> +    /*
> +     * Read the vtimer state directly from HVF. We're on the vCPU thread,
> +     * just exited from hv_vcpu_run(), so HVF holds the authoritative
> +     * values and env may be stale.
> +     */
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
> +    assert_hvf_ok(r);
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CVAL_EL0, &cval);
> +    assert_hvf_ok(r);
> +
> +    if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
> +        /* vtimer already expired, don't halt */
> +        return 0;
> +    }
> +
> +    cpu->halted = 1;
>      return EXCP_HLT;
>  }
>  
> @@ -2332,7 +2416,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>      hv_return_t r;
>  
>      if (cpu->halted) {
> -        return EXCP_HLT;
> +        if (!cpu_has_work(cpu)) {
> +            return EXCP_HLT;
> +        }
> +        cpu->halted = 0;
> +        timer_del(cpu->accel->wfi_timer);
>      }
>  
>      flush_cpu_state(cpu);
> @@ -2376,11 +2464,45 @@ static const VMStateDescription vmstate_hvf_vtimer = {
>  static void hvf_vm_state_change(void *opaque, bool running, RunState state)
>  {
>      HVFVTimer *s = opaque;
> +    CPUState *cpu;
>  
>      if (running) {
>          /* Update vtimer offset on all CPUs */
>          hvf_state->vtimer_offset = mach_absolute_time() - s->vtimer_val;
>          cpu_synchronize_all_states();
> +
> +        /*
> +         * After migration restore (or any resume), the wfi_timer is not
> +         * scheduled on this QEMU instance, so re-arm it for any halted
> +         * vCPU with a pending vtimer. For a non-migration resume the
> +         * QEMU_CLOCK_VIRTUAL timer was already scheduled; recomputing the
> +         * deadline produces the same value and is a harmless no-op.
> +         *
> +         * cpu_synchronize_all_states() above ensures env mirrors the
> +         * authoritative vtimer state (whether that came from HVF or from
> +         * the migration stream), so we can safely read it here from the
> +         * iothread.
> +         */
> +        CPU_FOREACH(cpu) {
> +            ARMCPU *arm_cpu;
> +            uint64_t ctl, cval;
> +
> +            if (!cpu->accel || !cpu->halted) {
> +                continue;
> +            }
> +
> +            arm_cpu = ARM_CPU(cpu);
> +            ctl = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].ctl;
> +            cval = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].cval;
> +
> +            if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
> +                /*
> +                 * vtimer already expired while we were paused; raise the
> +                 * IRQ now so the halted vCPU wakes up.
> +                 */
> +                hvf_wfi_timer_cb(cpu);
> +            }
> +        }
>      } else {
>          /* Remember vtimer value on every pause */
>          s->vtimer_val = hvf_vtimer_val_raw();

Hi Peter--

Sorry to nag, just was wondering if you could take a look at this follow-up
to your earlier review comments in:
https://lore.kernel.org/qemu-devel/CAFEAcA9RHQ+7++=kLn2goJwcgzDnaqdkQtQBtxQ2Rw1-uiKY=g@mail.gmail.com/#t

It's a bit of an unfortunate regression, so I was hoping to get some form of
the fix in.

Additionally, re: migration under hvf being broken, it  looks like one of my
fixes got merged, but there is one patch remaining here as well:
https://lore.kernel.org/qemu-devel/5e123304-628a-4da9-81ca-585498c809b6@linaro.org/

Thanks, and hope the ping is ok (just following the docs that say to retry
every 2 weeks)
-sjg

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Peter Maydell]

On Tue, 12 May 2026 at 17:46, Scott J. Goldman <scottjgo@gmail.com> wrote:
>
> On Mon Apr 27, 2026 at 3:55 PM EDT, Scott J. Goldman wrote:
> > Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> > changed hvf_wfi() from blocking the vCPU thread with pselect() to
> > returning EXCP_HLT, intending QEMU's main event loop to handle the
> > idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> > always returns false and the vCPU thread spins at 100% CPU per core
> > while the guest is idle.
> >
> > Fix this by:

> Hi Peter--
>
> Sorry to nag, just was wondering if you could take a look at this follow-up
> to your earlier review comments in:
> https://lore.kernel.org/qemu-devel/CAFEAcA9RHQ+7++=kLn2goJwcgzDnaqdkQtQBtxQ2Rw1-uiKY=g@mail.gmail.com/#t
>
> It's a bit of an unfortunate regression, so I was hoping to get some form of
> the fix in.

I'm really hoping that somebody who knows and uses hvf will
review this (and your other hvf patchset)...

-- PMM

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
> 
> Fix this by:
> 
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>   halt_cond in qemu_process_cpu_events().
> 
> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>   hv_vcpu_run(), which is not called while the CPU is halted. The
>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
> 
> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>   indicates a pending interrupt, and cancelling the WFI timer.
> 
> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>   transition for any halted vCPU, since the QEMUTimer is per-instance
>   state and is not migrated. After cpu_synchronize_all_states() the
>   migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>   and CNTV_CVAL from there. If the vtimer has already expired by the
>   time the destination resumes, hvf_wfi_timer_cb() is invoked
>   directly so the halted vCPU is woken up.
> 
> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>

Hi,

A bit of a side note for reproducing this:

To reproduce this on current master, as far as I can tell you need either 
-M kernel-irqchip=off or -M virt-11.0 or earlier. -M virt(-11.1) on master uses 
the HVF vGIC path.

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Tue May 12, 2026 at 1:54 PM EDT, Mohamed Mediouni wrote:
>
>
>> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
>> 
>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>> always returns false and the vCPU thread spins at 100% CPU per core
>> while the guest is idle.
>> 
>> Fix this by:
>> 
>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>   halt_cond in qemu_process_cpu_events().
>> 
>> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>>   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>   hv_vcpu_run(), which is not called while the CPU is halted. The
>>   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>   vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>> 
>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>   indicates a pending interrupt, and cancelling the WFI timer.
>> 
>> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>>   transition for any halted vCPU, since the QEMUTimer is per-instance
>>   state and is not migrated. After cpu_synchronize_all_states() the
>>   migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>>   and CNTV_CVAL from there. If the vtimer has already expired by the
>>   time the destination resumes, hvf_wfi_timer_cb() is invoked
>>   directly so the halted vCPU is woken up.
>> 
>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>
> Hi,
>
> A bit of a side note for reproducing this:
>
> To reproduce this on current master, as far as I can tell you need either 
> -M kernel-irqchip=off or -M virt-11.0 or earlier. -M virt(-11.1) on master uses 
> the HVF vGIC path.

Just wanted to confirm that I re-tested and it seems like you are
correct. This must have changed semi recently? I was just running with
`-M virt` before. Either way, it's good that the defaults seem to work
better now. Either way, I guess the fix still needs to go in for the
non vGIC path?

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 12. May 2026, at 22:36, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> On Tue May 12, 2026 at 1:54 PM EDT, Mohamed Mediouni wrote:
>> 
>> 
>>> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>> 
>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>> always returns false and the vCPU thread spins at 100% CPU per core
>>> while the guest is idle.
>>> 
>>> Fix this by:
>>> 
>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>>  halt_cond in qemu_process_cpu_events().
>>> 
>>> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>>>  virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>>  because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>>  hv_vcpu_run(), which is not called while the CPU is halted. The
>>>  timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>>  vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>>  interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>> 
>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>>  indicates a pending interrupt, and cancelling the WFI timer.
>>> 
>>> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>>>  transition for any halted vCPU, since the QEMUTimer is per-instance
>>>  state and is not migrated. After cpu_synchronize_all_states() the
>>>  migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>>>  and CNTV_CVAL from there. If the vtimer has already expired by the
>>>  time the destination resumes, hvf_wfi_timer_cb() is invoked
>>>  directly so the halted vCPU is woken up.
>>> 
>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>> 
>> Hi,
>> 
>> A bit of a side note for reproducing this:
>> 
>> To reproduce this on current master, as far as I can tell you need either 
>> -M kernel-irqchip=off or -M virt-11.0 or earlier. -M virt(-11.1) on master uses 
>> the HVF vGIC path.
> 
> Just wanted to confirm that I re-tested and it seems like you are
> correct. This must have changed semi recently?

Hi,

It finally got merged last week.

> I was just running with `-M virt` before. Either way, it's good that the defaults seem to work
> better now. Either way, I guess the fix still needs to go in for the
> non vGIC path?
> 

Yes. I’ll do a review. This patch is also a qemu-stable candidate.

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> changed hvf_wfi() from blocking the vCPU thread with pselect() to
> returning EXCP_HLT, intending QEMU's main event loop to handle the
> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
> always returns false and the vCPU thread spins at 100% CPU per core
> while the guest is idle.
> 
> Fix this by:
> 
> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>  halt_cond in qemu_process_cpu_events().
> 
> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>  virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>  because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>  hv_vcpu_run(), which is not called while the CPU is halted. The
>  timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>  vtimer IRQ through the GIC and marks vtimer_masked, causing the
>  interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
> 
> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>  indicates a pending interrupt, and cancelling the WFI timer.
> 
> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>  transition for any halted vCPU, since the QEMUTimer is per-instance
>  state and is not migrated. After cpu_synchronize_all_states() the
>  migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>  and CNTV_CVAL from there. If the vtimer has already expired by the
>  time the destination resumes, hvf_wfi_timer_cb() is invoked
>  directly so the halted vCPU is woken up.
> 
> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
> ---
> Changes since v2:
> - Use QEMU_CLOCK_VIRTUAL instead of QEMU_CLOCK_HOST so the timer
>  pauses with the VM and a halted vCPU isn't woken (or its IRQ
>  raised) while the user has stopped the guest. (Peter)
> - Convert vtimer ticks to nanoseconds with muldiv64() to avoid
>  intermediate overflow. (Peter)
> - Re-arm the WFI timer from hvf_vm_state_change() on the resume
>  transition so a halted vCPU on the migration destination is
>  woken when its vtimer expires (the QEMUTimer is per-instance
>  state and isn't migrated). (Peter)
> v2: https://lore.kernel.org/qemu-devel/20260410055045.63001-1-scottjgo@gmail.com/
> v1: https://lore.kernel.org/qemu-devel/20260410044726.61853-1-scottjgo@gmail.com/

For QEMU 11.0 (for backporting to stable):

Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>

For QEMU 11.1:

Adding some checks for !hvf_irqchip_in_kernel() needed
but can do them on my side if you prefer.

Looks ready apart from that bit.

> 
> include/system/hvf_int.h |   1 +
> target/arm/hvf/hvf.c     | 124 ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 124 insertions(+), 1 deletion(-)
> 
> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
> index 2621164cb2..58fb865eba 100644
> --- a/include/system/hvf_int.h
> +++ b/include/system/hvf_int.h
> @@ -48,6 +48,7 @@ struct AccelCPUState {
>    hv_vcpu_exit_t *exit;
>    bool vtimer_masked;
>    bool guest_debug_enabled;
> +    struct QEMUTimer *wfi_timer;
> #endif
> };
> 
> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> index 678afe5c8e..a19d7a5e1f 100644
> --- a/target/arm/hvf/hvf.c
> +++ b/target/arm/hvf/hvf.c
> @@ -28,6 +28,7 @@
> #include "hw/core/boards.h"
> #include "hw/core/irq.h"
> #include "qemu/main-loop.h"
> +#include "qemu/timer.h"
> #include "system/cpus.h"
> #include "arm-powerctl.h"
> #include "target/arm/cpu.h"
> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
> #define TMR_CTL_IMASK   (1 << 1)
> #define TMR_CTL_ISTATUS (1 << 2)
> 
> +static void hvf_wfi_timer_cb(void *opaque);
> +
> static uint32_t chosen_ipa_bit_size;
> 
> typedef struct HVFVTimer {
> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
> {
>    hv_return_t ret;
> 
> +    timer_free(cpu->accel->wfi_timer);
> +    cpu->accel->wfi_timer = NULL;
> +
>    ret = hv_vcpu_destroy(cpu->accel->fd);
>    assert_hvf_ok(ret);
> }
> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>                              arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>    assert_hvf_ok(ret);
> 
> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
> +                                          hvf_wfi_timer_cb, cpu);
> +
adding !hvf_irqchip_in_kernel()

>     aarch64_add_sme_properties(OBJECT(cpu));
>    return 0;
> }
> @@ -2027,8 +2036,67 @@ static uint64_t hvf_vtimer_val_raw(void)
>    return mach_absolute_time() - hvf_state->vtimer_offset;
> }
> 
> +static void hvf_wfi_timer_cb(void *opaque)
> +{
> +    CPUState *cpu = opaque;
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +
> +    /*
> +     * vtimer expired while the CPU was halted for WFI.
> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
> +     * interrupt and mark as masked so hvf_sync_vtimer() will
> +     * check and unmask when the guest handles it.
> +     *
> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
> +     */
> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
> +    cpu->accel->vtimer_masked = true;
> +}
> +
> +/*
> + * Arm a host-side QEMU_CLOCK_VIRTUAL timer to fire when the guest's
> + * vtimer (CNTV_CVAL_EL0) is scheduled to expire. HVF only delivers
> + * HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which we won't
> + * call while the vCPU is halted, so we need this to wake the vCPU.
> + *
> + * QEMU_CLOCK_VIRTUAL pauses while the VM is stopped, which keeps the
> + * timer in lockstep with the guest's view of vtime across pause/resume.
> + *
> + * Caller must supply the current CNTV_CTL_EL0 and CNTV_CVAL_EL0 values,
> + * since the appropriate source (HVF vs. env) depends on context.
> + *
> + * Returns 0 if the timer was armed (or if the vtimer is disabled/masked
> + * and the vCPU should still halt waiting on another event), or -1 if
> + * the vtimer has already expired.
> + */
> +static int hvf_arm_wfi_timer(CPUState *cpu, uint64_t ctl, uint64_t cval)
> +{
> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
> +    uint64_t now;
> +    int64_t delta_ns;
> +
> +    if (!(ctl & TMR_CTL_ENABLE) || (ctl & TMR_CTL_IMASK)) {
> +        return 0;
> +    }
> +
> +    now = hvf_vtimer_val_raw();
> +    if (cval <= now) {
> +        return -1;
> +    }
> +
> +    delta_ns = muldiv64(cval - now, NANOSECONDS_PER_SECOND,
> +                        arm_cpu->gt_cntfrq_hz);
> +    timer_mod(cpu->accel->wfi_timer,
> +              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + delta_ns);
> +    return 0;
> +}
> +
> static int hvf_wfi(CPUState *cpu)
> {
> +    uint64_t ctl, cval;
> +    hv_return_t r;
> +
>    if (cpu_has_work(cpu)) {
>        /*
>         * Don't bother to go into our "low power state" if
> @@ -2037,6 +2105,22 @@ static int hvf_wfi(CPUState *cpu)
>        return 0;
>    }
> 
> +    /*
> +     * Read the vtimer state directly from HVF. We're on the vCPU thread,
> +     * just exited from hv_vcpu_run(), so HVF holds the authoritative
> +     * values and env may be stale.
> +     */
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
> +    assert_hvf_ok(r);
> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CVAL_EL0, &cval);
> +    assert_hvf_ok(r);
> +
> +    if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
> +        /* vtimer already expired, don't halt */
> +        return 0;
> +    }
> +
> +    cpu->halted = 1;
>    return EXCP_HLT;
> }
> 
> @@ -2332,7 +2416,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>    hv_return_t r;
> 
>    if (cpu->halted) {
> -        return EXCP_HLT;
> +        if (!cpu_has_work(cpu)) {
> +            return EXCP_HLT;
> +        }
> +        cpu->halted = 0;
> +        timer_del(cpu->accel->wfi_timer);
!hvf_irqchip_in_kernel(), we shouldn’t have the wfi timer otherwise
>     }
> 
>    flush_cpu_state(cpu);
> @@ -2376,11 +2464,45 @@ static const VMStateDescription vmstate_hvf_vtimer = {
> static void hvf_vm_state_change(void *opaque, bool running, RunState state)
> {
>    HVFVTimer *s = opaque;
> +    CPUState *cpu;
> 
>    if (running) {
>        /* Update vtimer offset on all CPUs */
>        hvf_state->vtimer_offset = mach_absolute_time() - s->vtimer_val;
>        cpu_synchronize_all_states();
> +
> +        /*
> +         * After migration restore (or any resume), the wfi_timer is not
> +         * scheduled on this QEMU instance, so re-arm it for any halted
> +         * vCPU with a pending vtimer. For a non-migration resume the
> +         * QEMU_CLOCK_VIRTUAL timer was already scheduled; recomputing the
> +         * deadline produces the same value and is a harmless no-op.
> +         *
> +         * cpu_synchronize_all_states() above ensures env mirrors the
> +         * authoritative vtimer state (whether that came from HVF or from
> +         * the migration stream), so we can safely read it here from the
> +         * iothread.
> +         */

This should be gated behind !hvf_irqchip_in_kernel()

> +        CPU_FOREACH(cpu) {
> +            ARMCPU *arm_cpu;
> +            uint64_t ctl, cval;
> +
> +            if (!cpu->accel || !cpu->halted) {
> +                continue;
> +            }
> +
> +            arm_cpu = ARM_CPU(cpu);
> +            ctl = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].ctl;
> +            cval = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].cval;
> +
> +            if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
> +                /*
> +                 * vtimer already expired while we were paused; raise the
> +                 * IRQ now so the halted vCPU wakes up.
> +                 */
> +                hvf_wfi_timer_cb(cpu);
> +            }
> +        }
>    } else {
>        /* Remember vtimer value on every pause */
>        s->vtimer_val = hvf_vtimer_val_raw();
> -- 
> 2.50.1 (Apple Git-155)
> 
> 

[PATCH v11.1+ v4] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
changed hvf_wfi() from blocking the vCPU thread with pselect() to
returning EXCP_HLT, intending QEMU's main event loop to handle the
idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
always returns false and the vCPU thread spins at 100% CPU per core
while the guest is idle.

Fix this by:

1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
   halt_cond in qemu_process_cpu_events().

2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
   virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
   because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
   hv_vcpu_run(), which is not called while the CPU is halted. The
   timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
   vtimer IRQ through the GIC and marks vtimer_masked, causing the
   interrupt delivery chain to wake the vCPU via qemu_cpu_kick().

3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
   indicates a pending interrupt, and cancelling the WFI timer.

4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
   transition for any halted vCPU, since the QEMUTimer is per-instance
   state and is not migrated. After cpu_synchronize_all_states() the
   migrated vtimer state is mirrored in env, so we can read CNTV_CTL
   and CNTV_CVAL from there. If the vtimer has already expired by the
   time the destination resumes, hvf_wfi_timer_cb() is invoked
   directly so the halted vCPU is woken up.

All wfi_timer handling (allocation, arming, deletion, and the resume
re-arm) is gated on !hvf_irqchip_in_kernel(): with the Apple in-kernel
vGIC, HVF owns the vtimer and delivers wake-ups itself.

Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
---
v4: Gate all wfi_timer handling on !hvf_irqchip_in_kernel() (per
    Mohamed's review). This version is only intended for the `master`
    branch, while v3 can be applied to the 11.0 stable tree.
v3: Re-arm the wfi_timer from hvf_vm_state_change() so migration restore
    wakes a halted vCPU.

 include/system/hvf_int.h |   1 +
 target/arm/hvf/hvf.c     | 140 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 140 insertions(+), 1 deletion(-)

diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
index ad7d375109..d5eaf26dda 100644
--- a/include/system/hvf_int.h
+++ b/include/system/hvf_int.h
@@ -48,6 +48,7 @@ struct AccelCPUState {
     hv_vcpu_exit_t *exit;
     bool vtimer_masked;
     bool guest_debug_enabled;
+    struct QEMUTimer *wfi_timer;
 #endif
 };
 
diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
index 5a1718f7f9..0666175973 100644
--- a/target/arm/hvf/hvf.c
+++ b/target/arm/hvf/hvf.c
@@ -29,6 +29,7 @@
 #include "hw/core/irq.h"
 #include "hw/arm/virt.h"
 #include "qemu/main-loop.h"
+#include "qemu/timer.h"
 #include "system/cpus.h"
 #include "arm-powerctl.h"
 #include "target/arm/cpu.h"
@@ -308,6 +309,8 @@ void hvf_arm_init_debug(void)
 #define TMR_CTL_IMASK   (1 << 1)
 #define TMR_CTL_ISTATUS (1 << 2)
 
+static void hvf_wfi_timer_cb(void *opaque);
+
 static uint32_t chosen_ipa_bit_size;
 
 typedef struct HVFVTimer {
@@ -1296,6 +1299,11 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
 {
     hv_return_t ret;
 
+    if (!hvf_irqchip_in_kernel()) {
+        timer_free(cpu->accel->wfi_timer);
+        cpu->accel->wfi_timer = NULL;
+    }
+
     ret = hv_vcpu_destroy(cpu->accel->fd);
     assert_hvf_ok(ret);
 }
@@ -1494,6 +1502,11 @@ int hvf_arch_init_vcpu(CPUState *cpu)
                               arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
     assert_hvf_ok(ret);
 
+    if (!hvf_irqchip_in_kernel()) {
+        cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
+                                             hvf_wfi_timer_cb, cpu);
+    }
+
     aarch64_add_sme_properties(OBJECT(cpu));
     return 0;
 }
@@ -2201,6 +2214,62 @@ static uint64_t hvf_vtimer_val_raw(void)
     return mach_absolute_time() - hvf_state->vtimer_offset;
 }
 
+static void hvf_wfi_timer_cb(void *opaque)
+{
+    CPUState *cpu = opaque;
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+
+    /*
+     * vtimer expired while the CPU was halted for WFI.
+     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
+     * interrupt and mark as masked so hvf_sync_vtimer() will
+     * check and unmask when the guest handles it.
+     *
+     * The interrupt delivery chain (GIC -> cpu_interrupt ->
+     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
+     */
+    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
+    cpu->accel->vtimer_masked = true;
+}
+
+/*
+ * Arm a host-side QEMU_CLOCK_VIRTUAL timer to fire when the guest's
+ * vtimer (CNTV_CVAL_EL0) is scheduled to expire. HVF only delivers
+ * HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which we won't
+ * call while the vCPU is halted, so we need this to wake the vCPU.
+ *
+ * QEMU_CLOCK_VIRTUAL pauses while the VM is stopped, which keeps the
+ * timer in lockstep with the guest's view of vtime across pause/resume.
+ *
+ * Caller must supply the current CNTV_CTL_EL0 and CNTV_CVAL_EL0 values,
+ * since the appropriate source (HVF vs. env) depends on context.
+ *
+ * Returns 0 if the timer was armed (or if the vtimer is disabled/masked
+ * and the vCPU should still halt waiting on another event), or -1 if
+ * the vtimer has already expired.
+ */
+static int hvf_arm_wfi_timer(CPUState *cpu, uint64_t ctl, uint64_t cval)
+{
+    ARMCPU *arm_cpu = ARM_CPU(cpu);
+    uint64_t now;
+    int64_t delta_ns;
+
+    if (!(ctl & TMR_CTL_ENABLE) || (ctl & TMR_CTL_IMASK)) {
+        return 0;
+    }
+
+    now = hvf_vtimer_val_raw();
+    if (cval <= now) {
+        return -1;
+    }
+
+    delta_ns = muldiv64(cval - now, NANOSECONDS_PER_SECOND,
+                        arm_cpu->gt_cntfrq_hz);
+    timer_mod(cpu->accel->wfi_timer,
+              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + delta_ns);
+    return 0;
+}
+
 static int hvf_wfi(CPUState *cpu)
 {
     if (cpu_has_work(cpu)) {
@@ -2211,6 +2280,29 @@ static int hvf_wfi(CPUState *cpu)
         return 0;
     }
 
+    if (!hvf_irqchip_in_kernel()) {
+        uint64_t ctl, cval;
+        hv_return_t r;
+
+        /*
+         * Read the vtimer state directly from HVF. We're on the vCPU
+         * thread, just exited from hv_vcpu_run(), so HVF holds the
+         * authoritative values and env may be stale.
+         */
+        r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0,
+                                &ctl);
+        assert_hvf_ok(r);
+        r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CVAL_EL0,
+                                &cval);
+        assert_hvf_ok(r);
+
+        if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
+            /* vtimer already expired, don't halt */
+            return 0;
+        }
+    }
+
+    cpu->halted = 1;
     return EXCP_HLT;
 }
 
@@ -2509,7 +2601,13 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
     hv_return_t r;
 
     if (cpu->halted) {
-        return EXCP_HLT;
+        if (!cpu_has_work(cpu)) {
+            return EXCP_HLT;
+        }
+        cpu->halted = 0;
+        if (!hvf_irqchip_in_kernel()) {
+            timer_del(cpu->accel->wfi_timer);
+        }
     }
 
     flush_cpu_state(cpu);
@@ -2558,6 +2656,46 @@ static void hvf_vm_state_change(void *opaque, bool running, RunState state)
         /* Update vtimer offset on all CPUs */
         hvf_state->vtimer_offset = mach_absolute_time() - s->vtimer_val;
         cpu_synchronize_all_states();
+
+        /*
+         * After migration restore (or any resume), the wfi_timer is not
+         * scheduled on this QEMU instance, so re-arm it for any halted
+         * vCPU with a pending vtimer. For a non-migration resume the
+         * QEMU_CLOCK_VIRTUAL timer was already scheduled; recomputing the
+         * deadline produces the same value and is a harmless no-op.
+         *
+         * cpu_synchronize_all_states() above ensures env mirrors the
+         * authoritative vtimer state (whether that came from HVF or from
+         * the migration stream), so we can safely read it here from the
+         * iothread.
+         *
+         * Only applies when we own the wfi_timer; with an in-kernel vGIC
+         * the timer is never allocated and HVF handles vtimer wake-ups.
+         */
+        if (!hvf_irqchip_in_kernel()) {
+            CPUState *cpu;
+
+            CPU_FOREACH(cpu) {
+                ARMCPU *arm_cpu;
+                uint64_t ctl, cval;
+
+                if (!cpu->accel || !cpu->halted) {
+                    continue;
+                }
+
+                arm_cpu = ARM_CPU(cpu);
+                ctl = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].ctl;
+                cval = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].cval;
+
+                if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
+                    /*
+                     * vtimer already expired while we were paused; raise
+                     * the IRQ now so the halted vCPU wakes up.
+                     */
+                    hvf_wfi_timer_cb(cpu);
+                }
+            }
+        }
     } else {
         /* Remember vtimer value on every pause */
         s->vtimer_val = hvf_vtimer_val_raw();
-- 
2.50.1 (Apple Git-155)

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Scott J. Goldman]

On Tue May 12, 2026 at 9:21 PM EDT, Mohamed Mediouni wrote:
>
>> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
>> 
>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>> always returns false and the vCPU thread spins at 100% CPU per core
>> while the guest is idle.
>> 
>> Fix this by:
>> 
>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>  halt_cond in qemu_process_cpu_events().
>> 
>> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>>  virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>  because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>  hv_vcpu_run(), which is not called while the CPU is halted. The
>>  timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>  vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>  interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>> 
>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>  indicates a pending interrupt, and cancelling the WFI timer.
>> 
>> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>>  transition for any halted vCPU, since the QEMUTimer is per-instance
>>  state and is not migrated. After cpu_synchronize_all_states() the
>>  migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>>  and CNTV_CVAL from there. If the vtimer has already expired by the
>>  time the destination resumes, hvf_wfi_timer_cb() is invoked
>>  directly so the halted vCPU is woken up.
>> 
>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>> ---
>> Changes since v2:
>> - Use QEMU_CLOCK_VIRTUAL instead of QEMU_CLOCK_HOST so the timer
>>  pauses with the VM and a halted vCPU isn't woken (or its IRQ
>>  raised) while the user has stopped the guest. (Peter)
>> - Convert vtimer ticks to nanoseconds with muldiv64() to avoid
>>  intermediate overflow. (Peter)
>> - Re-arm the WFI timer from hvf_vm_state_change() on the resume
>>  transition so a halted vCPU on the migration destination is
>>  woken when its vtimer expires (the QEMUTimer is per-instance
>>  state and isn't migrated). (Peter)
>> v2: https://lore.kernel.org/qemu-devel/20260410055045.63001-1-scottjgo@gmail.com/
>> v1: https://lore.kernel.org/qemu-devel/20260410044726.61853-1-scottjgo@gmail.com/
>
> For QEMU 11.0 (for backporting to stable):
>
> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>
> For QEMU 11.1:
>
> Adding some checks for !hvf_irqchip_in_kernel() needed
> but can do them on my side if you prefer.
>
> Looks ready apart from that bit.

Hi Mohamed,

TFTR - I sent a follow-up patch that gates the wfi timer as you requested.
For applying this v3 version to the stable tree, is there anyone else I
should ping? I'm not very familiar with the process.
 
Thanks!
-sjg


>
>> 
>> include/system/hvf_int.h |   1 +
>> target/arm/hvf/hvf.c     | 124 ++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 124 insertions(+), 1 deletion(-)
>> 
>> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h
>> index 2621164cb2..58fb865eba 100644
>> --- a/include/system/hvf_int.h
>> +++ b/include/system/hvf_int.h
>> @@ -48,6 +48,7 @@ struct AccelCPUState {
>>    hv_vcpu_exit_t *exit;
>>    bool vtimer_masked;
>>    bool guest_debug_enabled;
>> +    struct QEMUTimer *wfi_timer;
>> #endif
>> };
>> 
>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
>> index 678afe5c8e..a19d7a5e1f 100644
>> --- a/target/arm/hvf/hvf.c
>> +++ b/target/arm/hvf/hvf.c
>> @@ -28,6 +28,7 @@
>> #include "hw/core/boards.h"
>> #include "hw/core/irq.h"
>> #include "qemu/main-loop.h"
>> +#include "qemu/timer.h"
>> #include "system/cpus.h"
>> #include "arm-powerctl.h"
>> #include "target/arm/cpu.h"
>> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)
>> #define TMR_CTL_IMASK   (1 << 1)
>> #define TMR_CTL_ISTATUS (1 << 2)
>> 
>> +static void hvf_wfi_timer_cb(void *opaque);
>> +
>> static uint32_t chosen_ipa_bit_size;
>> 
>> typedef struct HVFVTimer {
>> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)
>> {
>>    hv_return_t ret;
>> 
>> +    timer_free(cpu->accel->wfi_timer);
>> +    cpu->accel->wfi_timer = NULL;
>> +
>>    ret = hv_vcpu_destroy(cpu->accel->fd);
>>    assert_hvf_ok(ret);
>> }
>> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)
>>                              arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);
>>    assert_hvf_ok(ret);
>> 
>> +    cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
>> +                                          hvf_wfi_timer_cb, cpu);
>> +
> adding !hvf_irqchip_in_kernel()
>
>>     aarch64_add_sme_properties(OBJECT(cpu));
>>    return 0;
>> }
>> @@ -2027,8 +2036,67 @@ static uint64_t hvf_vtimer_val_raw(void)
>>    return mach_absolute_time() - hvf_state->vtimer_offset;
>> }
>> 
>> +static void hvf_wfi_timer_cb(void *opaque)
>> +{
>> +    CPUState *cpu = opaque;
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +
>> +    /*
>> +     * vtimer expired while the CPU was halted for WFI.
>> +     * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer
>> +     * interrupt and mark as masked so hvf_sync_vtimer() will
>> +     * check and unmask when the guest handles it.
>> +     *
>> +     * The interrupt delivery chain (GIC -> cpu_interrupt ->
>> +     * qemu_cpu_kick) wakes the vCPU thread from halt_cond.
>> +     */
>> +    qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);
>> +    cpu->accel->vtimer_masked = true;
>> +}
>> +
>> +/*
>> + * Arm a host-side QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>> + * vtimer (CNTV_CVAL_EL0) is scheduled to expire. HVF only delivers
>> + * HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which we won't
>> + * call while the vCPU is halted, so we need this to wake the vCPU.
>> + *
>> + * QEMU_CLOCK_VIRTUAL pauses while the VM is stopped, which keeps the
>> + * timer in lockstep with the guest's view of vtime across pause/resume.
>> + *
>> + * Caller must supply the current CNTV_CTL_EL0 and CNTV_CVAL_EL0 values,
>> + * since the appropriate source (HVF vs. env) depends on context.
>> + *
>> + * Returns 0 if the timer was armed (or if the vtimer is disabled/masked
>> + * and the vCPU should still halt waiting on another event), or -1 if
>> + * the vtimer has already expired.
>> + */
>> +static int hvf_arm_wfi_timer(CPUState *cpu, uint64_t ctl, uint64_t cval)
>> +{
>> +    ARMCPU *arm_cpu = ARM_CPU(cpu);
>> +    uint64_t now;
>> +    int64_t delta_ns;
>> +
>> +    if (!(ctl & TMR_CTL_ENABLE) || (ctl & TMR_CTL_IMASK)) {
>> +        return 0;
>> +    }
>> +
>> +    now = hvf_vtimer_val_raw();
>> +    if (cval <= now) {
>> +        return -1;
>> +    }
>> +
>> +    delta_ns = muldiv64(cval - now, NANOSECONDS_PER_SECOND,
>> +                        arm_cpu->gt_cntfrq_hz);
>> +    timer_mod(cpu->accel->wfi_timer,
>> +              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + delta_ns);
>> +    return 0;
>> +}
>> +
>> static int hvf_wfi(CPUState *cpu)
>> {
>> +    uint64_t ctl, cval;
>> +    hv_return_t r;
>> +
>>    if (cpu_has_work(cpu)) {
>>        /*
>>         * Don't bother to go into our "low power state" if
>> @@ -2037,6 +2105,22 @@ static int hvf_wfi(CPUState *cpu)
>>        return 0;
>>    }
>> 
>> +    /*
>> +     * Read the vtimer state directly from HVF. We're on the vCPU thread,
>> +     * just exited from hv_vcpu_run(), so HVF holds the authoritative
>> +     * values and env may be stale.
>> +     */
>> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);
>> +    assert_hvf_ok(r);
>> +    r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CVAL_EL0, &cval);
>> +    assert_hvf_ok(r);
>> +
>> +    if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
>> +        /* vtimer already expired, don't halt */
>> +        return 0;
>> +    }
>> +
>> +    cpu->halted = 1;
>>    return EXCP_HLT;
>> }
>> 
>> @@ -2332,7 +2416,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)
>>    hv_return_t r;
>> 
>>    if (cpu->halted) {
>> -        return EXCP_HLT;
>> +        if (!cpu_has_work(cpu)) {
>> +            return EXCP_HLT;
>> +        }
>> +        cpu->halted = 0;
>> +        timer_del(cpu->accel->wfi_timer);
> !hvf_irqchip_in_kernel(), we shouldn’t have the wfi timer otherwise
>>     }
>> 
>>    flush_cpu_state(cpu);
>> @@ -2376,11 +2464,45 @@ static const VMStateDescription vmstate_hvf_vtimer = {
>> static void hvf_vm_state_change(void *opaque, bool running, RunState state)
>> {
>>    HVFVTimer *s = opaque;
>> +    CPUState *cpu;
>> 
>>    if (running) {
>>        /* Update vtimer offset on all CPUs */
>>        hvf_state->vtimer_offset = mach_absolute_time() - s->vtimer_val;
>>        cpu_synchronize_all_states();
>> +
>> +        /*
>> +         * After migration restore (or any resume), the wfi_timer is not
>> +         * scheduled on this QEMU instance, so re-arm it for any halted
>> +         * vCPU with a pending vtimer. For a non-migration resume the
>> +         * QEMU_CLOCK_VIRTUAL timer was already scheduled; recomputing the
>> +         * deadline produces the same value and is a harmless no-op.
>> +         *
>> +         * cpu_synchronize_all_states() above ensures env mirrors the
>> +         * authoritative vtimer state (whether that came from HVF or from
>> +         * the migration stream), so we can safely read it here from the
>> +         * iothread.
>> +         */
>
> This should be gated behind !hvf_irqchip_in_kernel()
>
>> +        CPU_FOREACH(cpu) {
>> +            ARMCPU *arm_cpu;
>> +            uint64_t ctl, cval;
>> +
>> +            if (!cpu->accel || !cpu->halted) {
>> +                continue;
>> +            }
>> +
>> +            arm_cpu = ARM_CPU(cpu);
>> +            ctl = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].ctl;
>> +            cval = arm_cpu->env.cp15.c14_timer[GTIMER_VIRT].cval;
>> +
>> +            if (hvf_arm_wfi_timer(cpu, ctl, cval) < 0) {
>> +                /*
>> +                 * vtimer already expired while we were paused; raise the
>> +                 * IRQ now so the halted vCPU wakes up.
>> +                 */
>> +                hvf_wfi_timer_cb(cpu);
>> +            }
>> +        }
>>    } else {
>>        /* Remember vtimer value on every pause */
>>        s->vtimer_val = hvf_vtimer_val_raw();
>> -- 
>> 2.50.1 (Apple Git-155)
>> 
>> 

Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

[Mohamed Mediouni]

> On 13. May 2026, at 04:25, Scott J. Goldman <scottjgo@gmail.com> wrote:
> 
> On Tue May 12, 2026 at 9:21 PM EDT, Mohamed Mediouni wrote:
>> 
>>> On 27. Apr 2026, at 21:55, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>> 
>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>> always returns false and the vCPU thread spins at 100% CPU per core
>>> while the guest is idle.
>>> 
>>> Fix this by:
>>> 
>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>> halt_cond in qemu_process_cpu_events().
>>> 
>>> 2. Arming a per-vCPU QEMU_CLOCK_VIRTUAL timer to fire when the guest's
>>> virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>> because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>> hv_vcpu_run(), which is not called while the CPU is halted. The
>>> timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>> vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>> interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>> 
>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>> indicates a pending interrupt, and cancelling the WFI timer.
>>> 
>>> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>>> transition for any halted vCPU, since the QEMUTimer is per-instance
>>> state and is not migrated. After cpu_synchronize_all_states() the
>>> migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>>> and CNTV_CVAL from there. If the vtimer has already expired by the
>>> time the destination resumes, hvf_wfi_timer_cb() is invoked
>>> directly so the halted vCPU is woken up.
>>> 
>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>>> ---
>>> Changes since v2:
>>> - Use QEMU_CLOCK_VIRTUAL instead of QEMU_CLOCK_HOST so the timer
>>> pauses with the VM and a halted vCPU isn't woken (or its IRQ
>>> raised) while the user has stopped the guest. (Peter)
>>> - Convert vtimer ticks to nanoseconds with muldiv64() to avoid
>>> intermediate overflow. (Peter)
>>> - Re-arm the WFI timer from hvf_vm_state_change() on the resume
>>> transition so a halted vCPU on the migration destination is
>>> woken when its vtimer expires (the QEMUTimer is per-instance
>>> state and isn't migrated). (Peter)
>>> v2: https://lore.kernel.org/qemu-devel/20260410055045.63001-1-scottjgo@gmail.com/
>>> v1: https://lore.kernel.org/qemu-devel/20260410044726.61853-1-scottjgo@gmail.com/
>> 
>> For QEMU 11.0 (for backporting to stable):
>> 
>> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>> 
>> For QEMU 11.1:
>> 
>> Adding some checks for !hvf_irqchip_in_kernel() needed
>> but can do them on my side if you prefer.
>> 
>> Looks ready apart from that bit.
> 
> Hi Mohamed,
> 
> TFTR - I sent a follow-up patch that gates the wfi timer as you requested.
> For applying this v3 version to the stable tree, is there anyone else I
> should ping? I'm not very familiar with the process.
> 
> Thanks!
> -sjg
> 
> 

Hi,

v3 of this patch is for QEMU 11.0 qemu-stable and v4 is for QEMU 11.1.

A catch is that this isn’t in master yet though.

This regression doesn’t apply to QEMU 10.2 or earlier.

Could this regression fix be included into QEMU 11.0.1?

(with qemu-stable@ in cc)