Re: [XEN PATCH v12 2/7] x86/pvh: Allow (un)map_pirq when dom0 is PVH

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jul 31, 2024 at 11:55:35AM +0200, Jan Beulich wrote:
> On 31.07.2024 11:37, Roger Pau Monné wrote:
> > On Wed, Jul 31, 2024 at 11:02:01AM +0200, Jan Beulich wrote:
> >> On 31.07.2024 10:51, Roger Pau Monné wrote:
> >>> I agree with (a), but I don't think enabling PVH dom0 usage of the
> >>> hypercalls should be gated on this.  As said a PV dom0 is already
> >>> capable of issuing PHYSDEVOP_{,un}map_pirq operations against a PVH
> >>> domU.
> >>
> >> Okay, I can accept that as an intermediate position. We ought to deny
> >> such requests at some point though for PVH domains, the latest in the
> >> course of making vPCI work there.
> > 
> > Hm, once physdev_map_pirq() works as intended against PVH domains, I
> > don't see why we would prevent the usage of PHYSDEVOP_{,un}map_pirq
> > against such domains.
> 
> Well. If it can be made work as intended, then I certainly agree. However,
> without even the concept of pIRQ in PVH I'm having a hard time seeing how
> it can be made work. Iirc you were advocating for us to not introduce pIRQ
> into PVH.

From what I'm seeing here the intention is to expose
PHYSDEVOP_{,un}map_pirq to PVH dom0, so there must be some notion of
pIRQs or akin in a PVH dom0?  Even if only for passthrough needs.

> Maybe you're thinking of re-using the sub-ops, requiring PVH domains to
> pass in GSIs?

I think that was one my proposals, to either introduce a new
hypercall that takes a GSI, or to modify the PHYSDEVOP_{,un}map_pirq
in an ABI compatible way so that semantically the field could be a GSI
rather than a pIRQ.  We however would also need a way to reference an
MSI entry.

My main concern is not with pIRQs by itself, pIRQs are just an
abstract way to reference interrupts, my concern and what I wanted to
avoid on PVH is being able to route pIRQs over event channels.  IOW:
have interrupts from physical devices delivered over event channels.

> I think I suggested something along these lines also to
> Jiqian, yet with the now intended exposure to !has_pirq() domains I'm
> not sure this could be made work reliably.

I'm afraid I've been lacking behind on reviewing those series.

> Which reminds me of another question I had: What meaning does the pirq
> field have right now, if Dom0 would issue the request against a PVH DomU?
> What meaning will it have for a !has_pirq() HVM domain?

The pirq field could be a way to reference an interrupt.  It doesn't
need to be exposed to the PVH domU at all, but it's a way for the
device model to identify which interrupt should be mapped to which
domain.

Thanks, Roger.