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

On 2026/7/15 下午1:53, Tao Cui wrote:
> 
> 
> 在 2026/7/15 11:49, Bibo Mao 写道:
>>
>>
>> On 2026/7/15 上午11:28, Tao Cui wrote:
>>> From: Tao Cui <[email protected]>
>>>
>>> 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 <[email protected]>
>>> ---
>>>    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
The problem in _kvm_setcsr() is that there is only register 
LOONGARCH_CSR_TVAL however no context of LOONGARCH_CSR_TCFG register, it 
seems that it is better in to restore it in kvm_restore_timer() function 
where all registers about CPU timer are set.

How about something like this, it should work with period timer mode 
when migration.
@@ -119,6 +119,17 @@ void kvm_restore_timer(struct kvm_vcpu *vcpu)
         delta = 0;
         now = ktime_get();
         expire = vcpu->arch.expire;
+       if (!expire) {
+             /*
+              * The field vcpu->arch.expire is host-internal and is not
+              * migrated, so it is 0 after migration. Reload the
+              * remaining countdown from the migrated TVAL.
+              */
+               if (ticks < cfg)
+                       delta = tick_to_ns(vcpu, ticks);
+               expire = ktime_add_ns(ktime_get(), delta);
+       }
+
> rework the patch accordingly.
ok, that is fine. Thanks for catching this issue.

Regards
Bibo Mao
> 
> 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);
>>>
>>