Re: [RFC PATCH 00/19] GICv4 Support for Xen

[Julien Grall <julien@xxxxxxx>]

Hi Mykola,

On 13/02/2026 11:36, Mykola Kvach wrote:
> For this reason it may be worth conditionally recommending (or even
> auto-selecting) `vwfi=native` when direct injection is enabled for a
> vCPU, so measurements reflect the actual delivery fast-path rather than
> exit/scheduling overhead.
I don't think this is a straightforward answer. "vwfi=native" is 
beneficial when you have a single vCPU scheduled per pCPU. But if you 
have multiple vCPUs running, then you may impair the overall performance 
of the system as the scheduler will not be able to run another vCPU even 
if the current vCPU is doing nothing (it is waiting for an interrupt).
As a data point, Xen didn't initially trapped WFI/WFE. But we noticed a 
lot of slow down during boot if all the vCPUs for a guest were running 
on the same pCPU. The difference was quite noticeable.
So instead of recommending to always set "vwfi=native", I would consider 
an approach where Xen decides whether WFI/WFE is trapped based on the 
number of vCPUs that can be scheduled on a given pCPU. This could be 
adjusted on demand.
Cheers,

--
Julien Grall