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

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

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

> 
>> 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?

> 
> Jan

-- 
Best regards,
Jiqian Chen.