Re: [PATCH] LoongArch: KVM: Reload one-shot TVAL on migration destination

[Bibo Mao]

On 2026/7/15 上午11:28, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
>
> kvm_restore_timer() rebuilds the remaining timer countdown from
> vcpu->arch.expire, which is host-internal and is not part of the migrated
> vCPU state. On the migration destination it is still 0, so for a one-shot
> timer that has not expired yet the computed delta is 0 and
> write_gcsr_timertick(0) injects the timer interrupt immediately instead of
> after the remaining time. The guest observes a premature timer event right
> after migration.
>
> The expired one-shot case (TVAL = -1) is already handled earlier, so only
> the non-expired one-shot path is affected. When expire has not been set
> (i.e. on the destination), reload the remaining countdown from the
> migrated TVAL. The regular preempt/resume path on the source, where
> expire is valid, is unchanged.
>
> Fixes: a5857b9ff6e0 ("LoongArch: KVM: Implement vcpu timer operations")
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> ---
>   arch/loongarch/kvm/timer.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/arch/loongarch/kvm/timer.c b/arch/loongarch/kvm/timer.c
> index 3829f35a4070..57f53d19a00a 100644
> --- a/arch/loongarch/kvm/timer.c
> +++ b/arch/loongarch/kvm/timer.c
> @@ -132,6 +132,14 @@ void kvm_restore_timer(struct kvm_vcpu *vcpu)
>   		 * during injecting intr async
>   		 */
>   		kvm_queue_irq(vcpu, INT_TI);
> +	} else if (!expire) {
> +		/*
> +		 * One-shot timer on the migration destination: vcpu->arch.expire
> +		 * is host-internal and is not migrated, so it is still 0 here.
Good catch, I ever noticed this issue before however without good 
method. My previous method is to recalculate vcpu->arch.expire in 
function _kvm_setcsr() when LOONGARCH_CSR_TVAL is set and value of 
vcpu->arch.expire is 0.

Regards
Bibo Mao
> +		 * Reload the remaining countdown from the migrated TVAL instead
> +		 * of firing the timer immediately.
> +		 */
> +		delta = ticks;
>   	}
>   
>   	write_gcsr_timertick(delta);