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

- From: Cédric Le Goater \<clg@kaod.org\>
- Date: 2017-08-10 08:22:26 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2017/08/9499018
- Thread: https://ratatoskr.run/linuxppc-dev/2017/08/9498955/t.md

```text
On 08/10/2017 07:54 AM, David Gibson wrote:
> On Thu, Aug 10, 2017 at 02:46:00PM +1000, 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.
>>
>> Thus it's quite possible that using XIVE for IPIs is actually faster
>> than doorbells in that case.
> 
> Ok, well unless there's a compelling reason to use doorbells you can
> instead make it 3 cases with POWER9 never using doorbells.

yes I suppose we could improve things for XIVE. For XICS, there are 
another 3 backends behind icp_ops->cause_ipi. I need to take a look.

Thanks,

C. 
```