Re: [XEN PATCH v10 2/5] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.06.2024 08:49, Chen, Jiqian wrote:
> On 2024/6/17 22:45, Jan Beulich wrote:
>> On 17.06.2024 11:00, 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.
>>
>> Why "failed path"? Isn't unmapping also part of normal device removal
>> from a guest?
> Yes, both. I will change to also "allow PHYSDEVOP_unmap_pirq for the device 
> removal path to unmap pirq".
> 
>>
>>> And
>>> add a new check to prevent self map when subject domain has no
>>> PIRQ flag.
>>
>> You still talk of only self mapping, and the code also still does only
>> that. As pointed out before: Why would you allow mapping into a PVH
>> DomU? IOW what purpose do the "d == currd" checks have?
> The checking I added has two purpose, first is I need to allow this case:
> Dom0(without PIRQ) + DomU(with PIRQ), because the original code just do 
> (!has_pirq(currd)) will cause map_pirq fail in this case.
> Second I need to disallow self-mapping:
> DomU(without PIRQ) do map_pirq, the "d==currd" means the currd is the subject 
> domain itself.
> 
> Emmm, I think I know what's your concern.
> Do you mean I need to
> " Prevent map_pirq when currd has no X86_EMU_USE_PIRQ flag "
> instead of
> " Prevent self-map when currd has no X86_EMU_USE_PIRQ flag ",

No. What I mean is that I continue to fail to see why you mention "currd".
IOW it would be more like "prevent mapping when the subject domain has no
X86_EMU_USE_PIRQ" (which, as a specific sub-case, includes self-mapping
if the caller specifies DOMID_SELF for the subject domain).

> so I need to remove "d==currd", right?

Removing this check is what I'm after, yes. Yet that's not in sync with
either of the two quoted sentences above.

>>> So that domU with PIRQ flag can success to map pirq for
>>> passthrough devices even dom0 has no PIRQ flag.
>>
>> There's still a description problem here. Much like the first sentence,
>> this last one also says that the guest would itself map the pIRQ. In
>> which case there would still not be any reason to expose the sub-
>> functions to Dom0.
> If change to " So that the interrupt of a passthrough device can success to 
> be mapped to pirq for domU with PIRQ flag when dom0 is PVH.",
> Is it OK?

Kind of, yes. "can be successfully mapped" is one of the various possibilities
of making this read a little more smoothly.

Jan