Re: [PATCH 02/10] powerpc/xive: guest exploitation of the XIVE interrupt controller

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2017-08-10 at 08:45 +0200, Cédric Le Goater wrote:
> > The problem with doorbells on POWER9 guests is that they may have
> > to trap and be emulated by the hypervisor, since the guest threads
> > on P9 don't have to match the HW threads of the core.
> 
> Well, the pseries cause_ipi() handler does :
> 
>         if (doorbell_try_core_ipi(cpu))
>                 return;
> 
> to limit the doorbells to the same core. So we should be fine ?

No. It's theorically possible to create a guest that think it has 4
threads on P9 but those threads run on different cores of the host.

The doorbells are useful if KVM uses a "P8 style" whole-core dispatch
model or with PowerVM. We should probably invent some kind of DT
property to tell the guest I suppoes.

>  If not
> I suppose we should check CPU_FTR_ARCH_300 and use IPIs only for XIVE.
> 
> > Thus it's quite possible that using XIVE for IPIs is actually faster
> > than doorbells in that case.
> 
> How can we measure that ? ebizzy may be.

Or a simple socket ping pong with processes pinned to different
threads.

However the current KVM for P9 doesn't do threads yet afaik.

Cheers,
Ben.