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

- From: Cédric Le Goater \<clg@kaod.org\>
- Date: 2017-08-10 06:52:25 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2017/08/9499016
- Thread: https://ratatoskr.run/linuxppc-dev/2017/08/9498955/t.md

```text
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))
 		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.
```