Re: [PATCH] xen/arm: introduce vwfi parameter

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Tue, 2017-02-21 at 12:30 +0000, Julien Grall wrote:
> On 21/02/2017 09:09, Dario Faggioli wrote:
> > Well, TBH, we still are not entirely sure who the culprit is for
> > high
> > latency. There are spikes in Credit2, and I'm investigating that.
> > But
> > apart from them? I think we need other numbers with which we can
> > compare the numbers that Stefano has collected.
> 
> I think the problem is because we save/restore the vCPU state when 
> switching to the idle vCPU.
> 
That may well be. Or at least, that may well be part of the problem. I
don't know enough of ARM to know whether it's the predominant cause of
high latencies or not.

On x86, on Linux, polling is used to prevent the CPU to go in deep
C-states. I was assuming things to be similar on ARM, and that's the
reason why I thought introducing a polling mode could have been useful
(although wasteful).

But you guys are the ones that know whether or not that is the case
(and in another email, you seem to say it's not).

> Let say the only 1 vCPU can run on the pCPU, when the vCPU is issuing
> a 
> WFI the following steps will happen:
>       * WFI trapped and vcpu blocked
>       * save vCPU state
>       * run idle_loop
> -> Interrupt incoming for the guest
>       * restore vCPU state
>       * back to the guest
> 
> Saving/restoring on ARM requires to context switch all the state of
> the 
> VM (this is not saved in memory when entering in the hypervisor).
> This 
> include things like system register, interrupt controller state,
> FPU...
> 
Yes. In fact, on x86, we have what we call 'lazy context switch', which
deals specifically with some aspect of this situation.

Indeed it seems like implementing the same on ARM --if you don't have
it already, and if possible-- would be useful in this case too.

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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