Re: [Xen-devel] Fwd: [v3 14/15] Update Posted-Interrupts Descriptor during vCPU scheduling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.07.15 at 14:53, <george.dunlap@xxxxxxxxxxxxx> wrote:
>> The migration is handled in arch_pi_desc_update() which is called
>> by vcpu_runstate_change().
> 
> Well as far as I can tell from looking at the code,
> vcpu_runstate_change() will not be called when migrating a vcpu which is
> already blocked.
> 
> Consider the following scenario:
> - v1 blocks on pcpu 0.
>  - vcpu_runstate_change() will do everything necessary for v1 on p0.
> - The scheduler does load balancing and moves v1 to p1, calling
> vcpu_migrate().  Because the vcpu is still blocked,
> vcpu_runstate_change() is not called.
> - A device interrupt is generated.
> 
> What happens to the interrupt?  Does everything still work properly, or
> will the device wake-up interrupt go to the wrong pcpu (p0 rather than p1)?

I think much of this was discussed before, since I also disliked the
hooking into vcpu_runstate_change(). What I remember having
been told is that it really only matters which pCPU's list a vCPU is
on, not what v->processor says. And I sort of accepted that
moving the vCPU between lists when v->processor changes
without the vCPU actually running on that pCPU is pretty
pointless (and perhaps wasteful, but in any case cluttering more
code than necessary).

Jan


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