Re: [RFC XEN PATCH v8 5/5] domctl: Add XEN_DOMCTL_gsi_permission to grant gsi

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.05.2024 13:13, Chen, Jiqian wrote:
> On 2024/5/29 15:10, Jan Beulich wrote:
>> On 29.05.2024 08:56, Chen, Jiqian wrote:
>>> On 2024/5/29 14:31, Jan Beulich wrote:
>>>> On 29.05.2024 04:41, Chen, Jiqian wrote:
>>>>> But I found in function init_irq_data:
>>>>>     for ( irq = 0; irq < nr_irqs_gsi; irq++ )
>>>>>     {
>>>>>         int rc;
>>>>>
>>>>>         desc = irq_to_desc(irq);
>>>>>         desc->irq = irq;
>>>>>
>>>>>         rc = init_one_irq_desc(desc);
>>>>>         if ( rc )
>>>>>             return rc;
>>>>>     }
>>>>> Does it mean that when irq < nr_irqs_gsi, the gsi and irq is a 1:1 
>>>>> mapping?
>>>>
>>>> No, as explained before. I also don't see how you would derive that from 
>>>> the code above.
>>> Because here set desc->irq = irq, and it seems there is no other place to 
>>> change this desc->irq, so, gsi 1 is considered to irq 1.
>>
>> What are you taking this from? The loop bound isn't nr_gsis, and the 
>> iteration
>> variable isn't in GSI space either; it's in IRQ numbering space. In this loop
>> we're merely leveraging that every GSI has a corresponding IRQ;
>> there are no assumptions made about the mapping between the two. Afaics at 
>> least.
>>
>>>> "nr_irqs_gsi" describes what its name says: The number of
>>>> IRQs mapping to a (_some_) GSI. That's to tell them from the non-GSI (i.e.
>>>> mainly MSI) ones. There's no implication whatsoever on the IRQ <-> GSI
>>>> mapping.
>>>>
>>>>> What's more, when using PHYSDEVOP_setup_gsi, it calls mp_register_gsi,
>>>>> and in mp_register_gsi, it uses " desc = irq_to_desc(gsi); " to get 
>>>>> irq_desc directly.
>>>>
>>>> Which may be wrong, while that wrong-ness may not have hit anyone in
>>>> practice (for reasons that would need working out).
>>>>
>>>>> Combining above, can we consider "gsi == irq" when irq < nr_irqs_gsi ?
>>>>
>>>> Again - no.
>>> Since you are certain that they are not equal, could you tell me where show 
>>> they are not equal or where build their mappings,
>>> so that I can know how to do a conversion gsi from irq.
>>
>> I did point you at the ACPI Interrupt Source Override structure before.
>> We're parsing those in acpi_parse_int_src_ovr(), to give you a place to
>> start going from.
> Oh! I think I know.
> If I want to transform gsi to irq, I need to do below:
>       int irq, entry, ioapic, pin;
> 
>       ioapic = mp_find_ioapic(gsi);
>       pin = gsi - mp_ioapic_routing[ioapic].gsi_base;
>       entry = find_irq_entry(ioapic, pin, mp_INT);
>       irq = pin_2_irq(entry, ioapic, pin);
> 
> Am I right?

This looks plausible, yes.

Jan