Re: [RFC XEN PATCH v2 3/3] tools: Add new function to get gsi from irq

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

On 2023/11/29 00:11, Roger Pau Monné wrote:
> On Tue, Nov 28, 2023 at 03:42:31PM +0100, Juergen Gross wrote:
>> On 28.11.23 15:25, Roger Pau Monné wrote:
>>> On Fri, Nov 24, 2023 at 06:41:36PM +0800, Jiqian Chen wrote:
>>>> In PVH dom0, it uses the linux local interrupt mechanism,
>>>> when it allocs irq for a gsi, it is dynamic, and follow
>>>> the principle of applying first, distributing first. And
>>>> if you debug the kernel codes, you will find the irq
>>>> number is alloced from small to large, but the applying
>>>> gsi number is not, may gsi 38 comes before gsi 28, that
>>>> causes the irq number is not equal with the gsi number.
>>>> And when we passthrough a device, QEMU will use its gsi
>>>> number to do mapping actions, see xen_pt_realize->
>>>> xc_physdev_map_pirq, but the gsi number is got from file
>>>> /sys/bus/pci/devices/xxxx:xx:xx.x/irq in current code,
>>>> so it will fail when mapping.
>>>> And in current codes, there is no method to translate
>>>> irq to gsi for userspace.
>>>
>>> I think it would be cleaner to just introduce a new sysfs node that
>>> contains the gsi if a device is using one (much like the irq sysfs
>>> node)?
>>>
>>> Such ioctl to translate from IRQ to GSI has nothing to do with Xen, so
>>> placing it in privcmd does seem quite strange to me.  I understand
>>> that for passthrough we need the GSI, but such translation layer from
>>> IRQ to GSI is all Linux internal, and it would be much simpler to just
>>> expose the GSI in sysfs IMO.
>>
>> You are aware that we have a Xen specific variant of acpi_register_gsi()?
>>
>> It is the Xen event channel driver being responsible for the GSI<->IRQ
>> mapping.
> 
> I'm kind of lost, this translation function is specifically needed for
> PVH which doesn't use the Xen specific variant of acpi_register_gsi(),
> and hence the IRQ <-> GSI relation is whatever the Linux kernel does
> on native.
> 
> I do understand that on a PV dom0 the proposed sysfs gsi node would
> match the irq node, but that doesn't seem like an issue to me.
> 
> Note also that PVH doesn't use acpi_register_gsi_xen_hvm() because
> XENFEAT_hvm_pirqs feature is not exposed to PVH, so I expect it uses
> the x86 acpi_register_gsi_ioapic().
Yes, PVH use acpi_register_gsi_ioapic, thank Roger for explanation.

> 
> Thanks, Roger.

-- 
Best regards,
Jiqian Chen.