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Goldman" To: qemu-devel@nongnu.org Cc: qemu-arm@nongnu.org, Peter Maydell , Alexander Graf , Phil Dennis-Jordan , Roman Bolshakov , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , "Scott J. Goldman" Subject: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning Date: Mon, 27 Apr 2026 12:55:16 -0700 Message-ID: <20260427195516.46256-1-scottjgo@gmail.com> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260410055045.63001-1-scottjgo@gmail.com> References: <20260410055045.63001-1-scottjgo@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::122f; envelope-from=scottjgo@gmail.com; helo=mail-dl1-x122f.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-arm@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org Sender: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org 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 --- 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)