Re: [PATCH v1 08/15] xen/riscv: introduce vtimer_set_timer() and vtimer_expired()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.01.2026 13:27, Oleksii Kurochko wrote:
> On 1/13/26 4:12 PM, Jan Beulich wrote:
>> On 13.01.2026 15:44, Oleksii Kurochko wrote:
>>> On 1/8/26 11:28 AM, Jan Beulich wrote:
>>>> On 24.12.2025 18:03, Oleksii Kurochko wrote:
> Therefore, there is no real need to intercept accesses to these registers.

With this ...

>> you'd also need to synchronize both paths, I suppose.
> 
> I didn't get you what is needed to be synchronized. Could you please explain?

... there's nothing to synchronize.

>>>>> +    {
>>>>> +        stop_timer(&t->timer);
>>>>> +
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>>> +    set_timer(&t->timer, expires);
>>>> See the handling of VCPUOP_set_singleshot_timer for what you may want to
>>>> do if the expiry asked for is (perhaps just very slightly) into the past.
>>> I got an idea why we want to check if "expires" already expired, but ...
>>>
>>>> There you'll also find a use of migrate_timer(), which you will want to
>>>> at least consider using here as well.
>>> ... I don't get why we want to migrate timer before set_timer() here.
>>> Could you please explain that?
>> Didn't I see you use migrate_timer() in other patches (making me assume
>> you understand)? Having the timer tied to the pCPU where the vCPU runs
>> means the signalling to that vCPU will (commonly) be cheaper.
> 
> I thought that migrate_timer() is needed only when a vCPU changes the pCPU
> it is running on to ensure that it is running on correct pCPU after 
> migrations,
> hotplug events, or scheduling changes. That is why I placed it in
> vtimer_restore(), as there is no guarantee that the vCPU will run on the
> same pCPU it was running on previously.
> 
> So that is why ...
> 
>> Whether
>> that actually matters depends on what vtimer_expired() will eventually
>> contain. Hence why I said "consider using".
> 
> ... I didn't get why I might need vtimer_expired() in vtimer_set_timer()
> before set_timer().
> 
> vtimer_expired() will only notify the vCPU that a timer interrupt has
> occurred by setting bit in irqs_pending bitmap which then will be synced
> with vcpu->hvip, but I still do not understand whether migrate_timer()
> is needed before calling set_timer() here.

Just to repeat - it's not needed. It may be wanted.

> Considering that vtimer_set_timer() is called from the vCPU while it is
> running on the current pCPU, and assuming no pCPU rescheduling has
> occurred for this vCPU, we are already on the correct pCPU.
> If pCPU rescheduling for the vCPU did occur, then migrate_timer() would
> have been called in context_switch(),

Even if the timer wasn't active?

Jan

> and at the point where
> vtimer_set_timer() is invoked, we would already be running on the
> correct pCPU.
> 
> ~ Oleksii
>