Re: [Xen-devel] [PATCH] xen/arm: introduce vwfi parameter

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 21/02/17 17:49, Stefano Stabellini wrote:
> On Tue, 21 Feb 2017, Dario Faggioli wrote:
> > On Tue, 2017-02-21 at 13:46 +0000, George Dunlap wrote:
> > Oh, actually, if --which I only now realize may be what you are
> > referring to, since you're talking about "guest burning its credits"--
> > you let the vCPU put the pCPU to sleep *but*, when it wakes up (or when
> > the scheduler runs again for whatever reason), you charge to it for all
> > the time the the pCPU was actually idle/sleeping, well, that may
> > actually  not break scheduling, or cause disruption to the service of
> > other vCPUs.... But indeed I'd consider it rather counter intuitive a
> > behavior.
> How can this be safe? There could be no interrupts programmed to wake up
> the pcpu at all. In fact, I don't think today there would be any, unless
> we set one up in Xen for the specific purpose of interrupting the pcpu
> sleep.
>
> I don't know the inner working of the scheduler, but does it always send
> an interrupt to other pcpu to schedule something?
You still seem to assume that WFI/WFE is the only way to get a vCPU 
unscheduled. If that was the case it would be utterly wrong because you 
cannot expect a guest to use them.
> What if there are 2 vcpu pinned to the same pcpu? This cannot be fair.
Why wouldn't it be fair? This is the same situation as a guest vCPU not 
using WFI/WFE.
Cheers,

--
Julien Grall

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