Re: [XEN PATCH v9 2/5] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.06.2024 11:07, Chen, Jiqian wrote:
> On 2024/6/12 16:53, Jan Beulich wrote:
>> On 12.06.2024 04:43, Chen, Jiqian wrote:
>>> On 2024/6/10 23:58, Jan Beulich wrote:
>>>> On 07.06.2024 10:11, Jiqian Chen wrote:
>>>>> If run Xen with PVH dom0 and hvm domU, hvm will map a pirq for
>>>>> a passthrough device by using gsi, see qemu code
>>>>> xen_pt_realize->xc_physdev_map_pirq and libxl code
>>>>> pci_add_dm_done->xc_physdev_map_pirq. Then xc_physdev_map_pirq
>>>>> will call into Xen, but in hvm_physdev_op, PHYSDEVOP_map_pirq
>>>>> is not allowed because currd is PVH dom0 and PVH has no
>>>>> X86_EMU_USE_PIRQ flag, it will fail at has_pirq check.
>>>>>
>>>>> So, allow PHYSDEVOP_map_pirq when dom0 is PVH and also allow
>>>>> PHYSDEVOP_unmap_pirq for the failed path to unmap pirq. And
>>>>> add a new check to prevent self map when subject domain has no
>>>>> PIRQ flag.
>>>>>
>>>>> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
>>>>> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
>>>>> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>>
>>>> What's imo missing in the description is a clarification / justification of
>>>> why it is going to be a good idea (or at least an acceptable one) to expose
>>>> the concept of PIRQs to PVH. If I'm not mistaken that concept so far has
>>>> been entirely a PV one.
>>> I didn't want to expose the concept of PIRQs to PVH.
>>> I did this patch is for HVM that use PIRQs, what I said in commit message 
>>> is HVM will map a pirq for gsi, not PVH.
>>> For the original code, it checks " !has_pirq(currd)", but currd is PVH 
>>> dom0, so it failed. So I need to allow PHYSDEVOP_map_pirq
>>> even currd has no PIRQs, but the subject domain has.
>>
>> But that's not what you're enforcing in do_physdev_op(). There you only
>> prevent self-mapping. If I'm not mistaken all you need to do is drop the
>> "d == current->domain" checks from those conditionals.
> What I want is to allow PHYSDEVOP_map_pirq when currd doesn't have PIRQs, but 
> subject domain has.
> Then I just add "break" in hvm_physdev_op without any checks, that will cause 
> self-mapping problems.
> And in previous mail thread, you suggested me to prevent self-mapping when 
> subject domain doesn't have PIRQs.
> So I added checks in do_physdev_op.

Self-mapping was a primary concern of mine. Yet why deal with only a subset
of what needs preventing, when generalizing things actually can be done by
having less code.

Jan