[PATCH v2 0/2] Few timer and AIA fixes for KVM RISC-V

[PATCH v2 0/2] Few timer and AIA fixes for KVM RISC-V

[Anup Patel]

The RISC-V Privileged specificaiton says the following: "WFI is also
required to resume execution for locally enabled interrupts pending
at any privilege level, regardless of the global interrupt enable at
each privilege level."

Based on the above, if there is pending VS-timer interrupt when the
host (aka HS-mode) executes WFI then such a WFI will simply become NOP
and not do anything. This result in QEMU RISC-V consuming a lot of CPU
time on the x86 machine where it is running. The PATCH1 solves this
issue by adding appropriate cleanup in KVM RISC-V timer virtualization.

As a result PATCH1, race conditions in updating HGEI[E|P] CSRs when a
VCPU is moved from one host CPU to another are being observed on QEMU
so the PATCH2 tries to minimize the chances of these race conditions.

Changes since v1:
 - Added more details about race condition in PATCH2 commit description.

Anup Patel (2):
  RISC-V: KVM: Disable vstimecmp before exiting to user-space
  RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization

 arch/riscv/include/asm/kvm_aia.h |  4 ++-
 arch/riscv/kvm/aia.c             | 51 +++++---------------------------
 arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
 arch/riscv/kvm/vcpu.c            |  2 --
 arch/riscv/kvm/vcpu_timer.c      | 16 ++++++++++
 5 files changed, 71 insertions(+), 47 deletions(-)

-- 
2.43.0

[PATCH v2 1/2] RISC-V: KVM: Disable vstimecmp before exiting to user-space

[Anup Patel]

If VS-timer expires when no VCPU running on a host CPU then WFI
executed by such host CPU will be effective NOP resulting in no
power savings. This is as-per RISC-V Privileged specificaiton
which says: "WFI is also required to resume execution for locally
enabled interrupts pending at any privilege level, regardless of
the global interrupt enable at each privilege level."

To address the above issue, vstimecmp CSR must be set to -1UL over
here when VCPU is scheduled-out or exits to user space.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: 8f5cb44b1bae ("RISC-V: KVM: Support sstc extension")
Fixes: cea8896bd936 ("RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc")
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
---
 arch/riscv/kvm/vcpu_timer.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
index ff672fa71fcc..85a7262115e1 100644
--- a/arch/riscv/kvm/vcpu_timer.c
+++ b/arch/riscv/kvm/vcpu_timer.c
@@ -345,8 +345,24 @@ void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
 	/*
 	 * The vstimecmp CSRs are saved by kvm_riscv_vcpu_timer_sync()
 	 * upon every VM exit so no need to save here.
+	 *
+	 * If VS-timer expires when no VCPU running on a host CPU then
+	 * WFI executed by such host CPU will be effective NOP resulting
+	 * in no power savings. This is because as-per RISC-V Privileged
+	 * specificaiton: "WFI is also required to resume execution for
+	 * locally enabled interrupts pending at any privilege level,
+	 * regardless of the global interrupt enable at each privilege
+	 * level."
+	 *
+	 * To address the above issue, vstimecmp CSR must be set to -1UL
+	 * over here when VCPU is scheduled-out or exits to user space.
 	 */
 
+	csr_write(CSR_VSTIMECMP, -1UL);
+#if defined(CONFIG_32BIT)
+	csr_write(CSR_VSTIMECMPH, -1UL);
+#endif
+
 	/* timer should be enabled for the remaining operations */
 	if (unlikely(!t->init_done))
 		return;
-- 
2.43.0

[PATCH v2 2/2] RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization

[Anup Patel]

Currently, the common AIA functions kvm_riscv_vcpu_aia_has_interrupts()
and kvm_riscv_aia_wakeon_hgei() lookup HGEI line using an array of VCPU
pointers before accessing HGEI[E|P] CSR which is slow and prone to race
conditions because there is a separate per-hart lock for the VCPU pointer
array and a separate per-VCPU rwlock for IMSIC VS-file (including HGEI
line) used by the VCPU. Due to these race conditions, it is observed
on QEMU RISC-V host that Guest VCPUs sleep in WFI and never wakeup even
with interrupt pending in the IMSIC VS-file because VCPUs were waiting
for HGEI wakeup on the wrong host CPU.

The IMSIC virtualization already keeps track of the HGEI line and the
associated IMSIC VS-file used by each VCPU so move the HGEI[E|P] CSR
access to IMSIC virtualization so that costly HGEI line lookup can be
avoided and likelihood of race-conditions when updating HGEI[E|P] CSR
is also reduced.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: 3385339296d1 ("RISC-V: KVM: Use IMSIC guest files when available")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
---
 arch/riscv/include/asm/kvm_aia.h |  4 ++-
 arch/riscv/kvm/aia.c             | 51 +++++---------------------------
 arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
 arch/riscv/kvm/vcpu.c            |  2 --
 4 files changed, 55 insertions(+), 47 deletions(-)

diff --git a/arch/riscv/include/asm/kvm_aia.h b/arch/riscv/include/asm/kvm_aia.h
index 0a0f12496f00..b04ecdd1a860 100644
--- a/arch/riscv/include/asm/kvm_aia.h
+++ b/arch/riscv/include/asm/kvm_aia.h
@@ -87,6 +87,9 @@ DECLARE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
 
 extern struct kvm_device_ops kvm_riscv_aia_device_ops;
 
+bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu);
+void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu);
+void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu);
 void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu);
 int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu);
 
@@ -161,7 +164,6 @@ void kvm_riscv_aia_destroy_vm(struct kvm *kvm);
 int kvm_riscv_aia_alloc_hgei(int cpu, struct kvm_vcpu *owner,
 			     void __iomem **hgei_va, phys_addr_t *hgei_pa);
 void kvm_riscv_aia_free_hgei(int cpu, int hgei);
-void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable);
 
 void kvm_riscv_aia_enable(void);
 void kvm_riscv_aia_disable(void);
diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
index 19afd1f23537..dad318185660 100644
--- a/arch/riscv/kvm/aia.c
+++ b/arch/riscv/kvm/aia.c
@@ -30,28 +30,6 @@ unsigned int kvm_riscv_aia_nr_hgei;
 unsigned int kvm_riscv_aia_max_ids;
 DEFINE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
 
-static int aia_find_hgei(struct kvm_vcpu *owner)
-{
-	int i, hgei;
-	unsigned long flags;
-	struct aia_hgei_control *hgctrl = get_cpu_ptr(&aia_hgei);
-
-	raw_spin_lock_irqsave(&hgctrl->lock, flags);
-
-	hgei = -1;
-	for (i = 1; i <= kvm_riscv_aia_nr_hgei; i++) {
-		if (hgctrl->owners[i] == owner) {
-			hgei = i;
-			break;
-		}
-	}
-
-	raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
-
-	put_cpu_ptr(&aia_hgei);
-	return hgei;
-}
-
 static inline unsigned long aia_hvictl_value(bool ext_irq_pending)
 {
 	unsigned long hvictl;
@@ -95,7 +73,6 @@ void kvm_riscv_vcpu_aia_sync_interrupts(struct kvm_vcpu *vcpu)
 
 bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
 {
-	int hgei;
 	unsigned long seip;
 
 	if (!kvm_riscv_aia_available())
@@ -114,11 +91,7 @@ bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
 	if (!kvm_riscv_aia_initialized(vcpu->kvm) || !seip)
 		return false;
 
-	hgei = aia_find_hgei(vcpu);
-	if (hgei > 0)
-		return !!(ncsr_read(CSR_HGEIP) & BIT(hgei));
-
-	return false;
+	return kvm_riscv_vcpu_aia_imsic_has_interrupt(vcpu);
 }
 
 void kvm_riscv_vcpu_aia_update_hvip(struct kvm_vcpu *vcpu)
@@ -164,6 +137,9 @@ void kvm_riscv_vcpu_aia_load(struct kvm_vcpu *vcpu, int cpu)
 		csr_write(CSR_HVIPRIO2H, csr->hviprio2h);
 #endif
 	}
+
+	if (kvm_riscv_aia_initialized(vcpu->kvm))
+		kvm_riscv_vcpu_aia_imsic_load(vcpu, cpu);
 }
 
 void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
@@ -174,6 +150,9 @@ void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
 	if (!kvm_riscv_aia_available())
 		return;
 
+	if (kvm_riscv_aia_initialized(vcpu->kvm))
+		kvm_riscv_vcpu_aia_imsic_put(vcpu);
+
 	if (kvm_riscv_nacl_available()) {
 		nsh = nacl_shmem();
 		csr->vsiselect = nacl_csr_read(nsh, CSR_VSISELECT);
@@ -472,22 +451,6 @@ void kvm_riscv_aia_free_hgei(int cpu, int hgei)
 	raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
 }
 
-void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable)
-{
-	int hgei;
-
-	if (!kvm_riscv_aia_available())
-		return;
-
-	hgei = aia_find_hgei(owner);
-	if (hgei > 0) {
-		if (enable)
-			csr_set(CSR_HGEIE, BIT(hgei));
-		else
-			csr_clear(CSR_HGEIE, BIT(hgei));
-	}
-}
-
 static irqreturn_t hgei_interrupt(int irq, void *dev_id)
 {
 	int i;
diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
index ea1a36836d9c..fda0346f0ea1 100644
--- a/arch/riscv/kvm/aia_imsic.c
+++ b/arch/riscv/kvm/aia_imsic.c
@@ -677,6 +677,48 @@ static void imsic_swfile_update(struct kvm_vcpu *vcpu,
 	imsic_swfile_extirq_update(vcpu);
 }
 
+bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu)
+{
+	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
+	unsigned long flags;
+	bool ret = false;
+
+	/*
+	 * The IMSIC SW-file directly injects interrupt via hvip so
+	 * only check for interrupt when IMSIC VS-file is being used.
+	 */
+
+	read_lock_irqsave(&imsic->vsfile_lock, flags);
+	if (imsic->vsfile_cpu > -1)
+		ret = !!(csr_read(CSR_HGEIP) & BIT(imsic->vsfile_hgei));
+	read_unlock_irqrestore(&imsic->vsfile_lock, flags);
+
+	return ret;
+}
+
+void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu)
+{
+	/*
+	 * No need to explicitly clear HGEIE CSR bits because the
+	 * hgei interrupt handler (aka hgei_interrupt()) will always
+	 * clear it for us.
+	 */
+}
+
+void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu)
+{
+	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
+	unsigned long flags;
+
+	if (!kvm_vcpu_is_blocking(vcpu))
+		return;
+
+	read_lock_irqsave(&imsic->vsfile_lock, flags);
+	if (imsic->vsfile_cpu > -1)
+		csr_set(CSR_HGEIE, BIT(imsic->vsfile_hgei));
+	read_unlock_irqrestore(&imsic->vsfile_lock, flags);
+}
+
 void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu)
 {
 	unsigned long flags;
@@ -781,6 +823,9 @@ int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu)
 	 * producers to the new IMSIC VS-file.
 	 */
 
+	/* Ensure HGEIE CSR bit is zero before using the new IMSIC VS-file */
+	csr_clear(CSR_HGEIE, BIT(new_vsfile_hgei));
+
 	/* Zero-out new IMSIC VS-file */
 	imsic_vsfile_local_clear(new_vsfile_hgei, imsic->nr_hw_eix);
 
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index fe028b4274df..b26bf35a0a19 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -211,12 +211,10 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
 
 void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
 {
-	kvm_riscv_aia_wakeon_hgei(vcpu, true);
 }
 
 void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
 {
-	kvm_riscv_aia_wakeon_hgei(vcpu, false);
 }
 
 int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
-- 
2.43.0

Re: [PATCH v2 1/2] RISC-V: KVM: Disable vstimecmp before exiting to user-space

[Nutty Liu]

On 7/7/2025 11:53 AM, Anup Patel wrote:
> If VS-timer expires when no VCPU running on a host CPU then WFI
> executed by such host CPU will be effective NOP resulting in no
> power savings. This is as-per RISC-V Privileged specificaiton
> which says: "WFI is also required to resume execution for locally
> enabled interrupts pending at any privilege level, regardless of
> the global interrupt enable at each privilege level."
>
> To address the above issue, vstimecmp CSR must be set to -1UL over
> here when VCPU is scheduled-out or exits to user space.
>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>
> Tested-by: Atish Patra <atishp@rivosinc.com>
> Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Fixes: 8f5cb44b1bae ("RISC-V: KVM: Support sstc extension")
> Fixes: cea8896bd936 ("RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc")
> Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578
> Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> ---
>   arch/riscv/kvm/vcpu_timer.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
>
> diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> index ff672fa71fcc..85a7262115e1 100644
> --- a/arch/riscv/kvm/vcpu_timer.c
> +++ b/arch/riscv/kvm/vcpu_timer.c
> @@ -345,8 +345,24 @@ void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
>   	/*
>   	 * The vstimecmp CSRs are saved by kvm_riscv_vcpu_timer_sync()
>   	 * upon every VM exit so no need to save here.
> +	 *
> +	 * If VS-timer expires when no VCPU running on a host CPU then
> +	 * WFI executed by such host CPU will be effective NOP resulting
> +	 * in no power savings. This is because as-per RISC-V Privileged
> +	 * specificaiton: "WFI is also required to resume execution for
> +	 * locally enabled interrupts pending at any privilege level,
> +	 * regardless of the global interrupt enable at each privilege
> +	 * level."
> +	 *
> +	 * To address the above issue, vstimecmp CSR must be set to -1UL
> +	 * over here when VCPU is scheduled-out or exits to user space.
>   	 */
>   
> +	csr_write(CSR_VSTIMECMP, -1UL);
> +#if defined(CONFIG_32BIT)
> +	csr_write(CSR_VSTIMECMPH, -1UL);
> +#endif
> +
>   	/* timer should be enabled for the remaining operations */
>   	if (unlikely(!t->init_done))
>   		return;

Reviewed-by: Nutty Liu<liujingqi@lanxincomputing.com>

Thanks,
Nutty

Re: [PATCH v2 2/2] RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization

[Nutty Liu]

On 7/7/2025 11:53 AM, Anup Patel wrote:
> Currently, the common AIA functions kvm_riscv_vcpu_aia_has_interrupts()
> and kvm_riscv_aia_wakeon_hgei() lookup HGEI line using an array of VCPU
> pointers before accessing HGEI[E|P] CSR which is slow and prone to race
> conditions because there is a separate per-hart lock for the VCPU pointer
> array and a separate per-VCPU rwlock for IMSIC VS-file (including HGEI
> line) used by the VCPU. Due to these race conditions, it is observed
> on QEMU RISC-V host that Guest VCPUs sleep in WFI and never wakeup even
> with interrupt pending in the IMSIC VS-file because VCPUs were waiting
> for HGEI wakeup on the wrong host CPU.
>
> The IMSIC virtualization already keeps track of the HGEI line and the
> associated IMSIC VS-file used by each VCPU so move the HGEI[E|P] CSR
> access to IMSIC virtualization so that costly HGEI line lookup can be
> avoided and likelihood of race-conditions when updating HGEI[E|P] CSR
> is also reduced.
>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>
> Tested-by: Atish Patra <atishp@rivosinc.com>
> Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Fixes: 3385339296d1 ("RISC-V: KVM: Use IMSIC guest files when available")
> Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> ---
>   arch/riscv/include/asm/kvm_aia.h |  4 ++-
>   arch/riscv/kvm/aia.c             | 51 +++++---------------------------
>   arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
>   arch/riscv/kvm/vcpu.c            |  2 --
>   4 files changed, 55 insertions(+), 47 deletions(-)
>
> diff --git a/arch/riscv/include/asm/kvm_aia.h b/arch/riscv/include/asm/kvm_aia.h
> index 0a0f12496f00..b04ecdd1a860 100644
> --- a/arch/riscv/include/asm/kvm_aia.h
> +++ b/arch/riscv/include/asm/kvm_aia.h
> @@ -87,6 +87,9 @@ DECLARE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
>   
>   extern struct kvm_device_ops kvm_riscv_aia_device_ops;
>   
> +bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu);
> +void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu);
> +void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu);
>   void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu);
>   int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu);
>   
> @@ -161,7 +164,6 @@ void kvm_riscv_aia_destroy_vm(struct kvm *kvm);
>   int kvm_riscv_aia_alloc_hgei(int cpu, struct kvm_vcpu *owner,
>   			     void __iomem **hgei_va, phys_addr_t *hgei_pa);
>   void kvm_riscv_aia_free_hgei(int cpu, int hgei);
> -void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable);
>   
>   void kvm_riscv_aia_enable(void);
>   void kvm_riscv_aia_disable(void);
> diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
> index 19afd1f23537..dad318185660 100644
> --- a/arch/riscv/kvm/aia.c
> +++ b/arch/riscv/kvm/aia.c
> @@ -30,28 +30,6 @@ unsigned int kvm_riscv_aia_nr_hgei;
>   unsigned int kvm_riscv_aia_max_ids;
>   DEFINE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
>   
> -static int aia_find_hgei(struct kvm_vcpu *owner)
> -{
> -	int i, hgei;
> -	unsigned long flags;
> -	struct aia_hgei_control *hgctrl = get_cpu_ptr(&aia_hgei);
> -
> -	raw_spin_lock_irqsave(&hgctrl->lock, flags);
> -
> -	hgei = -1;
> -	for (i = 1; i <= kvm_riscv_aia_nr_hgei; i++) {
> -		if (hgctrl->owners[i] == owner) {
> -			hgei = i;
> -			break;
> -		}
> -	}
> -
> -	raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
> -
> -	put_cpu_ptr(&aia_hgei);
> -	return hgei;
> -}
> -
>   static inline unsigned long aia_hvictl_value(bool ext_irq_pending)
>   {
>   	unsigned long hvictl;
> @@ -95,7 +73,6 @@ void kvm_riscv_vcpu_aia_sync_interrupts(struct kvm_vcpu *vcpu)
>   
>   bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
>   {
> -	int hgei;
>   	unsigned long seip;
>   
>   	if (!kvm_riscv_aia_available())
> @@ -114,11 +91,7 @@ bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
>   	if (!kvm_riscv_aia_initialized(vcpu->kvm) || !seip)
>   		return false;
>   
> -	hgei = aia_find_hgei(vcpu);
> -	if (hgei > 0)
> -		return !!(ncsr_read(CSR_HGEIP) & BIT(hgei));
> -
> -	return false;
> +	return kvm_riscv_vcpu_aia_imsic_has_interrupt(vcpu);
>   }
>   
>   void kvm_riscv_vcpu_aia_update_hvip(struct kvm_vcpu *vcpu)
> @@ -164,6 +137,9 @@ void kvm_riscv_vcpu_aia_load(struct kvm_vcpu *vcpu, int cpu)
>   		csr_write(CSR_HVIPRIO2H, csr->hviprio2h);
>   #endif
>   	}
> +
> +	if (kvm_riscv_aia_initialized(vcpu->kvm))
> +		kvm_riscv_vcpu_aia_imsic_load(vcpu, cpu);
>   }
>   
>   void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
> @@ -174,6 +150,9 @@ void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
>   	if (!kvm_riscv_aia_available())
>   		return;
>   
> +	if (kvm_riscv_aia_initialized(vcpu->kvm))
> +		kvm_riscv_vcpu_aia_imsic_put(vcpu);
> +
>   	if (kvm_riscv_nacl_available()) {
>   		nsh = nacl_shmem();
>   		csr->vsiselect = nacl_csr_read(nsh, CSR_VSISELECT);
> @@ -472,22 +451,6 @@ void kvm_riscv_aia_free_hgei(int cpu, int hgei)
>   	raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
>   }
>   
> -void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable)
> -{
> -	int hgei;
> -
> -	if (!kvm_riscv_aia_available())
> -		return;
> -
> -	hgei = aia_find_hgei(owner);
> -	if (hgei > 0) {
> -		if (enable)
> -			csr_set(CSR_HGEIE, BIT(hgei));
> -		else
> -			csr_clear(CSR_HGEIE, BIT(hgei));
> -	}
> -}
> -
>   static irqreturn_t hgei_interrupt(int irq, void *dev_id)
>   {
>   	int i;
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index ea1a36836d9c..fda0346f0ea1 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -677,6 +677,48 @@ static void imsic_swfile_update(struct kvm_vcpu *vcpu,
>   	imsic_swfile_extirq_update(vcpu);
>   }
>   
> +bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu)
> +{
> +	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
> +	unsigned long flags;
> +	bool ret = false;
> +
> +	/*
> +	 * The IMSIC SW-file directly injects interrupt via hvip so
> +	 * only check for interrupt when IMSIC VS-file is being used.
> +	 */
> +
> +	read_lock_irqsave(&imsic->vsfile_lock, flags);
> +	if (imsic->vsfile_cpu > -1)
> +		ret = !!(csr_read(CSR_HGEIP) & BIT(imsic->vsfile_hgei));
> +	read_unlock_irqrestore(&imsic->vsfile_lock, flags);
> +
> +	return ret;
> +}
> +
> +void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu)
> +{
> +	/*
> +	 * No need to explicitly clear HGEIE CSR bits because the
> +	 * hgei interrupt handler (aka hgei_interrupt()) will always
> +	 * clear it for us.
> +	 */
> +}
> +
> +void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu)
> +{
> +	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
> +	unsigned long flags;
> +
> +	if (!kvm_vcpu_is_blocking(vcpu))
> +		return;
> +
> +	read_lock_irqsave(&imsic->vsfile_lock, flags);
> +	if (imsic->vsfile_cpu > -1)
> +		csr_set(CSR_HGEIE, BIT(imsic->vsfile_hgei));
> +	read_unlock_irqrestore(&imsic->vsfile_lock, flags);
> +}
> +
>   void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu)
>   {
>   	unsigned long flags;
> @@ -781,6 +823,9 @@ int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu)
>   	 * producers to the new IMSIC VS-file.
>   	 */
>   
> +	/* Ensure HGEIE CSR bit is zero before using the new IMSIC VS-file */
> +	csr_clear(CSR_HGEIE, BIT(new_vsfile_hgei));
> +
>   	/* Zero-out new IMSIC VS-file */
>   	imsic_vsfile_local_clear(new_vsfile_hgei, imsic->nr_hw_eix);
>   
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index fe028b4274df..b26bf35a0a19 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -211,12 +211,10 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
>   
>   void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
>   {
> -	kvm_riscv_aia_wakeon_hgei(vcpu, true);
>   }
>   
>   void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
>   {
> -	kvm_riscv_aia_wakeon_hgei(vcpu, false);
>   }
>   

Nitpick:
Should the above two empty functions be removed ?

Reviewed-by: Nutty Liu<liujingqi@lanxincomputing.com>

Thanks,
Nutty

>   int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)

Re: [PATCH v2 2/2] RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization

[Anup Patel]

On Mon, Jul 7, 2025 at 9:50 AM Nutty Liu <liujingqi@lanxincomputing.com> wrote:
>
> On 7/7/2025 11:53 AM, Anup Patel wrote:
> > Currently, the common AIA functions kvm_riscv_vcpu_aia_has_interrupts()
> > and kvm_riscv_aia_wakeon_hgei() lookup HGEI line using an array of VCPU
> > pointers before accessing HGEI[E|P] CSR which is slow and prone to race
> > conditions because there is a separate per-hart lock for the VCPU pointer
> > array and a separate per-VCPU rwlock for IMSIC VS-file (including HGEI
> > line) used by the VCPU. Due to these race conditions, it is observed
> > on QEMU RISC-V host that Guest VCPUs sleep in WFI and never wakeup even
> > with interrupt pending in the IMSIC VS-file because VCPUs were waiting
> > for HGEI wakeup on the wrong host CPU.
> >
> > The IMSIC virtualization already keeps track of the HGEI line and the
> > associated IMSIC VS-file used by each VCPU so move the HGEI[E|P] CSR
> > access to IMSIC virtualization so that costly HGEI line lookup can be
> > avoided and likelihood of race-conditions when updating HGEI[E|P] CSR
> > is also reduced.
> >
> > Reviewed-by: Atish Patra <atishp@rivosinc.com>
> > Tested-by: Atish Patra <atishp@rivosinc.com>
> > Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> > Fixes: 3385339296d1 ("RISC-V: KVM: Use IMSIC guest files when available")
> > Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> > ---
> >   arch/riscv/include/asm/kvm_aia.h |  4 ++-
> >   arch/riscv/kvm/aia.c             | 51 +++++---------------------------
> >   arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
> >   arch/riscv/kvm/vcpu.c            |  2 --
> >   4 files changed, 55 insertions(+), 47 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kvm_aia.h b/arch/riscv/include/asm/kvm_aia.h
> > index 0a0f12496f00..b04ecdd1a860 100644
> > --- a/arch/riscv/include/asm/kvm_aia.h
> > +++ b/arch/riscv/include/asm/kvm_aia.h
> > @@ -87,6 +87,9 @@ DECLARE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
> >
> >   extern struct kvm_device_ops kvm_riscv_aia_device_ops;
> >
> > +bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu);
> > +void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu);
> > +void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu);
> >   void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu);
> >   int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu);
> >
> > @@ -161,7 +164,6 @@ void kvm_riscv_aia_destroy_vm(struct kvm *kvm);
> >   int kvm_riscv_aia_alloc_hgei(int cpu, struct kvm_vcpu *owner,
> >                            void __iomem **hgei_va, phys_addr_t *hgei_pa);
> >   void kvm_riscv_aia_free_hgei(int cpu, int hgei);
> > -void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable);
> >
> >   void kvm_riscv_aia_enable(void);
> >   void kvm_riscv_aia_disable(void);
> > diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
> > index 19afd1f23537..dad318185660 100644
> > --- a/arch/riscv/kvm/aia.c
> > +++ b/arch/riscv/kvm/aia.c
> > @@ -30,28 +30,6 @@ unsigned int kvm_riscv_aia_nr_hgei;
> >   unsigned int kvm_riscv_aia_max_ids;
> >   DEFINE_STATIC_KEY_FALSE(kvm_riscv_aia_available);
> >
> > -static int aia_find_hgei(struct kvm_vcpu *owner)
> > -{
> > -     int i, hgei;
> > -     unsigned long flags;
> > -     struct aia_hgei_control *hgctrl = get_cpu_ptr(&aia_hgei);
> > -
> > -     raw_spin_lock_irqsave(&hgctrl->lock, flags);
> > -
> > -     hgei = -1;
> > -     for (i = 1; i <= kvm_riscv_aia_nr_hgei; i++) {
> > -             if (hgctrl->owners[i] == owner) {
> > -                     hgei = i;
> > -                     break;
> > -             }
> > -     }
> > -
> > -     raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
> > -
> > -     put_cpu_ptr(&aia_hgei);
> > -     return hgei;
> > -}
> > -
> >   static inline unsigned long aia_hvictl_value(bool ext_irq_pending)
> >   {
> >       unsigned long hvictl;
> > @@ -95,7 +73,6 @@ void kvm_riscv_vcpu_aia_sync_interrupts(struct kvm_vcpu *vcpu)
> >
> >   bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
> >   {
> > -     int hgei;
> >       unsigned long seip;
> >
> >       if (!kvm_riscv_aia_available())
> > @@ -114,11 +91,7 @@ bool kvm_riscv_vcpu_aia_has_interrupts(struct kvm_vcpu *vcpu, u64 mask)
> >       if (!kvm_riscv_aia_initialized(vcpu->kvm) || !seip)
> >               return false;
> >
> > -     hgei = aia_find_hgei(vcpu);
> > -     if (hgei > 0)
> > -             return !!(ncsr_read(CSR_HGEIP) & BIT(hgei));
> > -
> > -     return false;
> > +     return kvm_riscv_vcpu_aia_imsic_has_interrupt(vcpu);
> >   }
> >
> >   void kvm_riscv_vcpu_aia_update_hvip(struct kvm_vcpu *vcpu)
> > @@ -164,6 +137,9 @@ void kvm_riscv_vcpu_aia_load(struct kvm_vcpu *vcpu, int cpu)
> >               csr_write(CSR_HVIPRIO2H, csr->hviprio2h);
> >   #endif
> >       }
> > +
> > +     if (kvm_riscv_aia_initialized(vcpu->kvm))
> > +             kvm_riscv_vcpu_aia_imsic_load(vcpu, cpu);
> >   }
> >
> >   void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
> > @@ -174,6 +150,9 @@ void kvm_riscv_vcpu_aia_put(struct kvm_vcpu *vcpu)
> >       if (!kvm_riscv_aia_available())
> >               return;
> >
> > +     if (kvm_riscv_aia_initialized(vcpu->kvm))
> > +             kvm_riscv_vcpu_aia_imsic_put(vcpu);
> > +
> >       if (kvm_riscv_nacl_available()) {
> >               nsh = nacl_shmem();
> >               csr->vsiselect = nacl_csr_read(nsh, CSR_VSISELECT);
> > @@ -472,22 +451,6 @@ void kvm_riscv_aia_free_hgei(int cpu, int hgei)
> >       raw_spin_unlock_irqrestore(&hgctrl->lock, flags);
> >   }
> >
> > -void kvm_riscv_aia_wakeon_hgei(struct kvm_vcpu *owner, bool enable)
> > -{
> > -     int hgei;
> > -
> > -     if (!kvm_riscv_aia_available())
> > -             return;
> > -
> > -     hgei = aia_find_hgei(owner);
> > -     if (hgei > 0) {
> > -             if (enable)
> > -                     csr_set(CSR_HGEIE, BIT(hgei));
> > -             else
> > -                     csr_clear(CSR_HGEIE, BIT(hgei));
> > -     }
> > -}
> > -
> >   static irqreturn_t hgei_interrupt(int irq, void *dev_id)
> >   {
> >       int i;
> > diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> > index ea1a36836d9c..fda0346f0ea1 100644
> > --- a/arch/riscv/kvm/aia_imsic.c
> > +++ b/arch/riscv/kvm/aia_imsic.c
> > @@ -677,6 +677,48 @@ static void imsic_swfile_update(struct kvm_vcpu *vcpu,
> >       imsic_swfile_extirq_update(vcpu);
> >   }
> >
> > +bool kvm_riscv_vcpu_aia_imsic_has_interrupt(struct kvm_vcpu *vcpu)
> > +{
> > +     struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
> > +     unsigned long flags;
> > +     bool ret = false;
> > +
> > +     /*
> > +      * The IMSIC SW-file directly injects interrupt via hvip so
> > +      * only check for interrupt when IMSIC VS-file is being used.
> > +      */
> > +
> > +     read_lock_irqsave(&imsic->vsfile_lock, flags);
> > +     if (imsic->vsfile_cpu > -1)
> > +             ret = !!(csr_read(CSR_HGEIP) & BIT(imsic->vsfile_hgei));
> > +     read_unlock_irqrestore(&imsic->vsfile_lock, flags);
> > +
> > +     return ret;
> > +}
> > +
> > +void kvm_riscv_vcpu_aia_imsic_load(struct kvm_vcpu *vcpu, int cpu)
> > +{
> > +     /*
> > +      * No need to explicitly clear HGEIE CSR bits because the
> > +      * hgei interrupt handler (aka hgei_interrupt()) will always
> > +      * clear it for us.
> > +      */
> > +}
> > +
> > +void kvm_riscv_vcpu_aia_imsic_put(struct kvm_vcpu *vcpu)
> > +{
> > +     struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
> > +     unsigned long flags;
> > +
> > +     if (!kvm_vcpu_is_blocking(vcpu))
> > +             return;
> > +
> > +     read_lock_irqsave(&imsic->vsfile_lock, flags);
> > +     if (imsic->vsfile_cpu > -1)
> > +             csr_set(CSR_HGEIE, BIT(imsic->vsfile_hgei));
> > +     read_unlock_irqrestore(&imsic->vsfile_lock, flags);
> > +}
> > +
> >   void kvm_riscv_vcpu_aia_imsic_release(struct kvm_vcpu *vcpu)
> >   {
> >       unsigned long flags;
> > @@ -781,6 +823,9 @@ int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu)
> >        * producers to the new IMSIC VS-file.
> >        */
> >
> > +     /* Ensure HGEIE CSR bit is zero before using the new IMSIC VS-file */
> > +     csr_clear(CSR_HGEIE, BIT(new_vsfile_hgei));
> > +
> >       /* Zero-out new IMSIC VS-file */
> >       imsic_vsfile_local_clear(new_vsfile_hgei, imsic->nr_hw_eix);
> >
> > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > index fe028b4274df..b26bf35a0a19 100644
> > --- a/arch/riscv/kvm/vcpu.c
> > +++ b/arch/riscv/kvm/vcpu.c
> > @@ -211,12 +211,10 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
> >
> >   void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
> >   {
> > -     kvm_riscv_aia_wakeon_hgei(vcpu, true);
> >   }
> >
> >   void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
> >   {
> > -     kvm_riscv_aia_wakeon_hgei(vcpu, false);
> >   }
> >
>
> Nitpick:
> Should the above two empty functions be removed ?
>

We cannot remove these functions but we can certainly
make them static inline. I will update in the next revision.

Thanks,
Anup

Re: [PATCH v2 0/2] Few timer and AIA fixes for KVM RISC-V

[Anup Patel]

On Mon, Jul 7, 2025 at 9:23 AM Anup Patel <apatel@ventanamicro.com> wrote:
>
> The RISC-V Privileged specificaiton says the following: "WFI is also
> required to resume execution for locally enabled interrupts pending
> at any privilege level, regardless of the global interrupt enable at
> each privilege level."
>
> Based on the above, if there is pending VS-timer interrupt when the
> host (aka HS-mode) executes WFI then such a WFI will simply become NOP
> and not do anything. This result in QEMU RISC-V consuming a lot of CPU
> time on the x86 machine where it is running. The PATCH1 solves this
> issue by adding appropriate cleanup in KVM RISC-V timer virtualization.
>
> As a result PATCH1, race conditions in updating HGEI[E|P] CSRs when a
> VCPU is moved from one host CPU to another are being observed on QEMU
> so the PATCH2 tries to minimize the chances of these race conditions.
>
> Changes since v1:
>  - Added more details about race condition in PATCH2 commit description.
>
> Anup Patel (2):
>   RISC-V: KVM: Disable vstimecmp before exiting to user-space
>   RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization
>
>  arch/riscv/include/asm/kvm_aia.h |  4 ++-
>  arch/riscv/kvm/aia.c             | 51 +++++---------------------------
>  arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
>  arch/riscv/kvm/vcpu.c            |  2 --
>  arch/riscv/kvm/vcpu_timer.c      | 16 ++++++++++
>  5 files changed, 71 insertions(+), 47 deletions(-)
>

Queued this series as fixes for Linux-6.16

I have taken care of the comment on PATCH2 at the time of queuing.

Thanks,
Anup

Re: [PATCH v2 0/2] Few timer and AIA fixes for KVM RISC-V

[patchwork-bot+linux-riscv]

Hello:

This series was applied to riscv/linux.git (for-next)
by Anup Patel <anup@brainfault.org>:

On Mon,  7 Jul 2025 09:23:42 +0530 you wrote:
> The RISC-V Privileged specificaiton says the following: "WFI is also
> required to resume execution for locally enabled interrupts pending
> at any privilege level, regardless of the global interrupt enable at
> each privilege level."
> 
> Based on the above, if there is pending VS-timer interrupt when the
> host (aka HS-mode) executes WFI then such a WFI will simply become NOP
> and not do anything. This result in QEMU RISC-V consuming a lot of CPU
> time on the x86 machine where it is running. The PATCH1 solves this
> issue by adding appropriate cleanup in KVM RISC-V timer virtualization.
> 
> [...]

Here is the summary with links:
  - [v2,1/2] RISC-V: KVM: Disable vstimecmp before exiting to user-space
    https://git.kernel.org/riscv/c/57f576e860d3
  - [v2,2/2] RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization
    (no matching commit)

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
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