Re: [RFC v2 1/6] xen/arm: Save and restore support with hvm context hypercalls

[Julien Grall <julien.grall@linaro.org>]

On 05/12/2014 10:16 AM, Ian Campbell wrote:
> On Thu, 2014-04-17 at 16:06 +0100, Julien Grall wrote:
>> I thought a bit more about the {phys,virt}_timer_base.offset.
>>
>> When you are migrating a guest, this offset will be invalidated. This
>> offset is used to get a relative offset from the Xen timer counter.
>>
>> That also made me think the context switch in Xen for the timer looks
>> wrong to me.
>>
>> When a guest VCPU is context switch, the Xen timer counter continue to
>> run. But not CVAL, so the timer_base.offset will drift a bit. It will
>> result by setting a wrong timer via set_timer in Xen.
>>
>> Did I miss something?
> 
> The timer offset is mainly accounting for the fact that the domain is
> not booted when the hardware is started.
> 
> However time does continue while a VCPU is not scheduled, this is
> exposed via the PV "stolen time" mechanism.
> 
> Now it is in theory possible to virtualise time differently so that
> stolen time is not possible, but unless you want to cope with different
> VCPUs seeing different times (because they have been descheduled for
> differently lengths of times) then you either need to do gang scheduling
> or play other (likely complicated) tricks. With the model we have on ARM
> paravirtualising this is the right thing to do.
> 
> Not sure what you mean about CVAL (the timer compare val) not running,
> when we deschedule a VCPU we figure out when CVAL would have caused the
> timer interrupt to fire and setup a Xen timer to make sure we unblock
> the VCPU at that point. When we switch back to the VCPU we of course
> restore the compare value to what the guest wrote, nothing else would
> make sense.

After reading your explanation and the ARM ARM again, I think I mingled
CNT (the counter) and CVAL (the compare val).

Thank you for the explanation.

-- 
Julien Grall