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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.07.2024 13:29, Roger Pau Monné wrote:
> 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.

Only in so far as it is an abstract, handle-like value pertaining solely
to the target domain.

>> 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.

Of course.

> 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.

Oh, I might have slightly misunderstood your intentions then.

>> 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.

Since pIRQ-s are per-domain, _that_ kind of association won't be
helped. But yes, as per above it could serve as an abstract handle-
like value.

Jan