Re: [RFC KERNEL PATCH v2 2/3] xen/pvh: Unmask irq for passthrough device in PVH dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.12.2023 12:18, Roger Pau Monné wrote:
> On Tue, Dec 12, 2023 at 10:38:08AM +0100, Jan Beulich wrote:
>> (I think the Cc list is too long here, but then I don't know who to
>> keep and who to possibly drop.)
>>
>> On 12.12.2023 09:49, Roger Pau Monné wrote:
>>> On Tue, Dec 12, 2023 at 06:16:43AM +0000, Chen, Jiqian wrote:
>>>> On 2023/12/11 23:45, Roger Pau Monné wrote:
>>>>> On Wed, Dec 06, 2023 at 06:07:26AM +0000, Chen, Jiqian wrote:
>>>>>> +static int xen_pvh_setup_gsi(gsi_info_t *gsi_info)
>>>>>> +{
>>>>>> +       struct physdev_setup_gsi setup_gsi;
>>>>>> +
>>>>>> +       setup_gsi.gsi = gsi_info->gsi;
>>>>>> +       setup_gsi.triggering = (gsi_info->trigger == ACPI_EDGE_SENSITIVE 
>>>>>> ? 0 : 1);
>>>>>> +       setup_gsi.polarity = (gsi_info->polarity == ACPI_ACTIVE_HIGH ? 0 
>>>>>> : 1);
>>>>>> +
>>>>>> +       return HYPERVISOR_physdev_op(PHYSDEVOP_setup_gsi, &setup_gsi);
>>>>>> +}
>>>>>
>>>>> Hm, why not simply call pcibios_enable_device() from pciback?  What
>>>> pcibios_enable_device had been called when using cmd "xl 
>>>> pci-assignable-add sbdf" from pciback. But it didn't do map_pirq and 
>>>> setup_gsi.
>>>> Because pcibios_enable_device-> pcibios_enable_irq-> 
>>>> __acpi_register_gsi(acpi_register_gsi_ioapic PVH specific)
>>>>> you are doing here using the hypercalls is a backdoor into what's done
>>>>> automatically by Xen on IO-APIC accesses by a PVH dom0.
>>>> But the gsi didn't be unmasked, and vioapic_hwdom_map_gsi is never called.
>>>> So, I think in pciback, if we can do what vioapic_hwdom_map_gsi does.
>>>>
>>>
>>> I see, it does setup the IO-APIC pin but doesn't unmask it, that's
>>> what I feared.
>>>
>>>>> It will be much more natural for the PVH dom0 model to simply use the
>>>>> native way to configure and unmask the IO-APIC pin, and that would
>>>>> correctly setup the triggering/polarity and bind it to dom0 without
>>>>> requiring the usage of any hypercalls.
>>>> Do you still prefer that I called unmask_irq in pcistub_init_device, as 
>>>> this v2 patch do?
>>>> But Thomas Gleixner think it is not suitable to export unmask_irq.
>>>
>>> Yeah, that wasn't good.
>>>
>>>>>
>>>>> Is that an issue since in that case the gsi will get mapped and bound
>>>>> to dom0?
>>>> Dom0 do map_pirq is to pass the check xc_domain_irq_permission()-> 
>>>> pirq_access_permitted(), 
>>>
>>> Can we see about finding another way to fix this check?
>>>
>>> One option would be granting permissions over the IRQ in
>>> PHYSDEVOP_setup_gsi?
>>
>> There's no domain available there, and imo it's also the wrong interface to
>> possibly grant any permissions.
> 
> Well, the domain is the caller.

Granting permission to itself?

Jan