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

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2024/7/31 21:03, Roger Pau Monné wrote:
> On Wed, Jul 31, 2024 at 01:39:40PM +0200, Jan Beulich wrote:
>> 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.
> 
> My intention would be to not even use pIRQs at all, in order to avoid
> the temptation of the guest itself managing interrupts using
> hypercalls, hence I would have preferred that abstract interface to be
> something else.
> 
> Maybe we could even expose the Xen IRQ space directly, and just use
> that as interrupt handles, but since I'm not the one doing the work
> I'm not sure it's fair to ask for something that would require more
> changes internally to Xen.
> 
>>>> 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.
> 
> I would be fine with doing the interrupt bindings based on IRQs
> instead of pIRQs, but I'm afraid that would require more changes to
> hypercalls and Xen internals.
> 
> At some point I need to work on a new interface to do passthrough, so
> that we can remove the usage of domctls from QEMU.  That might be a
> good opportunity to switch from using pIRQs.

Thanks for your input, but I may be a bit behind you with my knowledge and 
can't fully understand the discussion.
How should I modify this question later?
Should I add a new hypercall specifically for passthrough?
Or if it is to prevent the (un)map from being used for PVH guests, can I just 
add a new function to check if the subject domain is a PVH type? Like 
is_pvh_domain().

> 
> Thanks, Roger.

-- 
Best regards,
Jiqian Chen.