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

[Tao Cui]

在 2026/7/15 11:49, Bibo Mao 写道:
> 
> 
> 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.
> 
Thanks for the review and for sharing your idea. I also considered
recalculating expire in _kvm_setcsr() when TVAL is restored, but ended
up keeping the change within kvm_restore_timer() to avoid touching the
core CSR write path. Both approaches are functionally equivalent since
the first save_timer() after restore will set expire correctly either
way.

That said, if you prefer the _kvm_setcsr() approach, I'm happy to
rework the patch accordingly.

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