Re: [Xen-devel] [PATCH RFC v2] Add SUPPORT.md

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 23 Oct 2017, Andrew Cooper wrote:
> >>> +### x86 PV/Event Channels
> >>> +
> >>> +    Limit: 131072
> >> Why do we call out event channel limits but not grant table limits? 
> >> Also, why is this x86?  The 2l and fifo ABIs are arch agnostic, as far
> >> as I am aware.
> > Sure, but I'm pretty sure that ARM guests don't (perhaps cannot?) use PV
> > event channels.
> 
> This is mixing the hypervisor API/ABI capabilities with the actual
> abilities of guests (which is also different to what Linux would use in
> the guests).
> 
> ARM guests, as well as x86 HVM with APICV (configured properly) will
> actively want to avoid the guest event channel interface, because its
> slower.
> 
> This solitary evtchn limit serves no useful purpose IMO.

Just a clarification: ARM guests have event channels. They are delivered
to the guest using a single PPI (per processor interrupt). I am pretty
sure that limit on the number of event channels on ARM is the same as on
x86 because they both depend on the same fifo ABI.

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