[PATCH] RISC-V: KVM: Fix timer state restore

[PATCH] RISC-V: KVM: Fix timer state restore

[Qiang Ma]

The KVM_REG_RISCV_TIMER_REG(state) one-reg write passes the value
written by userspace to kvm_riscv_vcpu_timer_next_event() when
re-enabling the timer.

That value is the timer state, KVM_RISCV_TIMER_STATE_ON, not the
timer compare value. During migration or state restore, userspace
restores the compare register separately, which stores the target
cycle in t->next_cycles. Re-arming the timer with the state value
schedules the next event at cycle 1 instead of the restored compare
value, causing the virtual timer to fire too early.

Use the restored compare value from t->next_cycles when turning the
timer back on.

Fixes: 3a9f66cb25e1 ("RISC-V: KVM: Add timer functionality")

Signed-off-by: Qiang Ma <maqianga@uniontech.com>
---
 arch/riscv/kvm/vcpu_timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
index 9817ff802821..ae53133c7ab0 100644
--- a/arch/riscv/kvm/vcpu_timer.c
+++ b/arch/riscv/kvm/vcpu_timer.c
@@ -231,7 +231,7 @@ int kvm_riscv_vcpu_set_reg_timer(struct kvm_vcpu *vcpu,
 		break;
 	case KVM_REG_RISCV_TIMER_REG(state):
 		if (reg_val == KVM_RISCV_TIMER_STATE_ON)
-			ret = kvm_riscv_vcpu_timer_next_event(vcpu, reg_val);
+			ret = kvm_riscv_vcpu_timer_next_event(vcpu, t->next_cycles);
 		else
 			ret = kvm_riscv_vcpu_timer_cancel(t);
 		break;
-- 
2.20.1


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Re: [PATCH] RISC-V: KVM: Fix timer state restore

[Anup Patel]

On Tue, May 26, 2026 at 1:26 PM Qiang Ma <maqianga@uniontech.com> wrote:
>
> The KVM_REG_RISCV_TIMER_REG(state) one-reg write passes the value
> written by userspace to kvm_riscv_vcpu_timer_next_event() when
> re-enabling the timer.
>
> That value is the timer state, KVM_RISCV_TIMER_STATE_ON, not the
> timer compare value. During migration or state restore, userspace
> restores the compare register separately, which stores the target
> cycle in t->next_cycles. Re-arming the timer with the state value
> schedules the next event at cycle 1 instead of the restored compare
> value, causing the virtual timer to fire too early.
>
> Use the restored compare value from t->next_cycles when turning the
> timer back on.
>
> Fixes: 3a9f66cb25e1 ("RISC-V: KVM: Add timer functionality")
>
> Signed-off-by: Qiang Ma <maqianga@uniontech.com>

LGTM.

Reviewed-by: Anup Patel <anup@brainfault.org>

Queued this patch for Linux-7.2

Thanks,
Anup

> ---
>  arch/riscv/kvm/vcpu_timer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> index 9817ff802821..ae53133c7ab0 100644
> --- a/arch/riscv/kvm/vcpu_timer.c
> +++ b/arch/riscv/kvm/vcpu_timer.c
> @@ -231,7 +231,7 @@ int kvm_riscv_vcpu_set_reg_timer(struct kvm_vcpu *vcpu,
>                 break;
>         case KVM_REG_RISCV_TIMER_REG(state):
>                 if (reg_val == KVM_RISCV_TIMER_STATE_ON)
> -                       ret = kvm_riscv_vcpu_timer_next_event(vcpu, reg_val);
> +                       ret = kvm_riscv_vcpu_timer_next_event(vcpu, t->next_cycles);
>                 else
>                         ret = kvm_riscv_vcpu_timer_cancel(t);
>                 break;
> --
> 2.20.1
>

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Re: [PATCH] RISC-V: KVM: Fix timer state restore

[patchwork-bot+linux-riscv]

Hello:

This patch was applied to riscv/linux.git (fixes)
by Anup Patel <anup@brainfault.org>:

On Tue, 26 May 2026 15:55:44 +0800 you wrote:
> The KVM_REG_RISCV_TIMER_REG(state) one-reg write passes the value
> written by userspace to kvm_riscv_vcpu_timer_next_event() when
> re-enabling the timer.
> 
> That value is the timer state, KVM_RISCV_TIMER_STATE_ON, not the
> timer compare value. During migration or state restore, userspace
> restores the compare register separately, which stores the target
> cycle in t->next_cycles. Re-arming the timer with the state value
> schedules the next event at cycle 1 instead of the restored compare
> value, causing the virtual timer to fire too early.
> 
> [...]

Here is the summary with links:
  - RISC-V: KVM: Fix timer state restore
    https://git.kernel.org/riscv/c/6d4c17ed5620

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