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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.08.2024 10:11, Roger Pau Monné wrote:
> On Fri, Aug 02, 2024 at 02:37:24AM +0000, Chen, Jiqian wrote:
>> 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().
> 
> I think that would be part of a new interface, as said before I don't
> think it would be fair to force you to do all this work.  I won't
> oppose with the approach to attempt to re-use the existing interfaces
> as much as possible.
> 
> I think this patch needs to be adjusted to drop the change to
> xen/arch/x86/physdev.c, as just allowing PHYSDEVOP_{,un}map_pirq
> without any change to do_physdev_op() should result in the correct
> behavior.

Plus perhaps adding respective clarification to the description, as
to exposing the functionality to wider than (presently) necessary
"audience".

Jan