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.

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

On 7/4/25 8:38 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.
>
> 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: Atish Patra <atishp@rivosinc.com>

On 04.07.25 17:38, 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.
> 
> 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>

Anup and Atish, thanks a lot for investigating.

I built upstream kernel 6.14.9 with the patches of this series and that 
resolved the problem reported in Launchpad bug report 2112578.

Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.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;