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

(Adding back Xen devel)

On 05/13/2014 04:31 PM, Wei Huang wrote:
> On 05/12/2014 07:04 AM, Julien Grall wrote:
>> 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.
> Other than the code comments (case/switch), are you OK with the design
> of the latest ARCH_TIMER patch?

I made some comment on the v3. Once you will address comments from
Andrew and me, the patch will be in good shape.


Julien Grall

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