Re: [XEN PATCH v14 2/5] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.09.2024 09:58, Chen, Jiqian wrote:
> On 2024/9/3 15:42, Jan Beulich wrote:
>> On 03.09.2024 09:04, Jiqian Chen wrote:
>>> When dom0 is PVH type and passthrough a device to HVM domU, Qemu code
>>> xen_pt_realize->xc_physdev_map_pirq and libxl code pci_add_dm_done->
>>> xc_physdev_map_pirq map a pirq for passthrough devices.
>>> In xc_physdev_map_pirq call stack, function hvm_physdev_op has a check
>>> has_pirq(currd), but currd is PVH dom0, PVH has no X86_EMU_USE_PIRQ flag,
>>> so it fails, PHYSDEVOP_map_pirq is not allowed for PVH dom0 in current
>>> codes.
>>>
>>> But it is fine to map interrupts through pirq to a HVM domain whose
>>> XENFEAT_hvm_pirqs is not enabled. Because pirq field is used as a way to
>>> reference interrupts and it is just the way for the device model to
>>> identify which interrupt should be mapped to which domain, however
>>> has_pirq() is just to check if HVM domains route interrupts from
>>> devices(emulated or passthrough) through event channel, so, the has_pirq()
>>> check should not be applied to the PHYSDEVOP_map_pirq issued by dom0.
>>>
>>> So, allow PHYSDEVOP_map_pirq when dom0 is PVH and also allow
>>> PHYSDEVOP_unmap_pirq for the removal device path to unmap pirq. Then the
>>> interrupt of a passthrough device can be successfully mapped to pirq for 
>>> domU.
>>
>> As before: When you talk about just Dom0, ...
>>
>>> --- a/xen/arch/x86/hvm/hypercall.c
>>> +++ b/xen/arch/x86/hvm/hypercall.c
>>> @@ -73,6 +73,8 @@ long hvm_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>>> arg)
>>>      {
>>>      case PHYSDEVOP_map_pirq:
>>>      case PHYSDEVOP_unmap_pirq:
>>> +        break;
>>> +
>>>      case PHYSDEVOP_eoi:
>>>      case PHYSDEVOP_irq_status_query:
>>>      case PHYSDEVOP_get_free_pirq:
>>
>> ... that ought to match the code. IOW you've again lost why it is okay(ish)
>> (or even necessary) to also permit the op for non-Dom0 (e.g. a PVH stubdom).
>> Similarly imo Dom0 using this on itself wants discussing.
> Do you mean I need to talk about why permit this op for all HVM

You don't need to invent reasons, but it needs making clear that wider than
necessary (for your purpose) exposure is at least not going to be a problem.

> and  where can prevent PVH domain calling this for self-mapping, not only 
> dom0?

Aiui use on itself is limited to Dom0, so only that would need clarifying
(along the lines of the above, i.e. that/why it is not a problem). For
has_pirq() domains use on oneself was already permitted before.

Jan