Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Tue, 2015-09-22 at 15:15 +0100, George Dunlap wrote:
> On 09/22/2015 02:52 PM, Wu, Feng wrote:
> > 
> > 
> > > -----Original Message-----
> > > From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]

> > > Yes, the idle to vCPUB switch is covered by __context_switch(),
> > > but
> > it cannot change the PI state of vCPUA at that point. Like
> > mentioned
> > above, in this case, spurious PI interrupts happens.
> 
> On the contrary, when __context_switch() is called for the idle ->
> vcpuB
> transition in the above scenario, it is *actually* context switching
> from vcpuA to vcpuB, since vcpuA is still actually on the cpu.  
>  Which
> means that if you add PI code into arch->ctxt_switch_from(), the case
> you describe above will be handled automatically.
> 
Yep, that's exactly what I was also writing... luckily, I've seen
George's email before getting too far with that! :-P

That's the point of lazy context switch. The context of vCPUA is still
on pCPUA during the time idle executes. If, at the next transition
point, we switch from idle to vCPUA again, then nothing is needed, and
we just saved one context saving and one context restoring operation.
If it is something else, like in your case, we need to save vCPUA's
context, which is actually what we have on pCPUA, despite idle (and not
vCPUA itself) was what was running.

> So the only downside to doing everything in block(), wake(), and
> __context_switch() is that if a VM is offline, or preempted by a
> tasklet, and an interrupt comes in, we will get a spurious PI (i.e.,
> one
> which interrupts us but we can't do anything useful about).
> 
Indeed. And that also seems bearable to me. Feng, are there reasons why
you think it's not?

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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