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

[Julien Grall <julien.grall@xxxxxxx>]

On 21/02/2017 18:30, Stefano Stabellini wrote:
> On Tue, 21 Feb 2017, Julien Grall wrote:
> > 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.
> I read your suggestion as trapping WFI in Xen, then, depending on
> settings, executing WFI in the Xen trap handler to idle the pcpu. That
> doesn't work. But I take you suggested not trapping wfi (remove
> HCR_TWI), executing the instruction in guest context. That is what we
> used to do in the early days (before a780f750). It should be safe and
> possibly even quick. I'll rerun the numbers and let you know.
My first suggestion was to emulate WFI in Xen, which I agree is not safe :).

I think not trapping WFI will have the best performance but may impact 
the credit of the vCPU as mentioned by Dario and George.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel