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

On 08/10/2017 06:46 AM, Benjamin Herrenschmidt wrote:

On Thu, 2017-08-10 at 14:28 +1000, David Gibson wrote:

quoted

Also, will POWER9 always have doorbells?  In which case you could
reduce it to 3 options.

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 ? 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.

C.