On 08/10/2017 06:46 AM, Benjamin Herrenschmidt wrote:
> On Thu, 2017-08-10 at 14:28 +1000, David Gibson wrote:
>> 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))

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.


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