Re: [RFC XEN PATCH 6/6] tools/libs/light: pci: translate irq to gsi

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.03.2023 01:44, Stefano Stabellini wrote:
> On Wed, 15 Mar 2023, Roger Pau Monné wrote:
>> On Sun, Mar 12, 2023 at 03:54:55PM +0800, Huang Rui wrote:
>>> From: Chen Jiqian <Jiqian.Chen@xxxxxxx>
>>>
>>> Use new xc_physdev_gsi_from_irq to get the GSI number
>>>
>>> Signed-off-by: Chen Jiqian <Jiqian.Chen@xxxxxxx>
>>> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
>>> ---
>>>  tools/libs/light/libxl_pci.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/tools/libs/light/libxl_pci.c b/tools/libs/light/libxl_pci.c
>>> index f4c4f17545..47cf2799bf 100644
>>> --- a/tools/libs/light/libxl_pci.c
>>> +++ b/tools/libs/light/libxl_pci.c
>>> @@ -1486,6 +1486,7 @@ static void pci_add_dm_done(libxl__egc *egc,
>>>          goto out_no_irq;
>>>      }
>>>      if ((fscanf(f, "%u", &irq) == 1) && irq) {
>>> +        irq = xc_physdev_gsi_from_irq(ctx->xch, irq);
>>
>> This is just a shot in the dark, because I don't really have enough
>> context to understand what's going on here, but see below.
>>
>> I've taken a look at this on my box, and it seems like on
>> dom0 the value returned by /sys/bus/pci/devices/SBDF/irq is not
>> very consistent.
>>
>> If devices are in use by a driver the irq sysfs node reports either
>> the GSI irq or the MSI IRQ (in case a single MSI interrupt is
>> setup).
>>
>> It seems like pciback in Linux does something to report the correct
>> value:
>>
>> root@lcy2-dt107:~# cat /sys/bus/pci/devices/0000\:00\:14.0/irq
>> 74
>> root@lcy2-dt107:~# xl pci-assignable-add 00:14.0
>> root@lcy2-dt107:~# cat /sys/bus/pci/devices/0000\:00\:14.0/irq
>> 16
>>
>> As you can see, making the device assignable changed the value
>> reported by the irq node to be the GSI instead of the MSI IRQ, I would
>> think you are missing something similar in the PVH setup (some pciback
>> magic)?
>>
>> Albeit I have no idea why you would need to translate from IRQ to GSI
>> in the way you do in this and related patches, because I'm missing the
>> context.
> 
> As I mention in another email, also keep in mind that we need QEMU to
> work and QEMU calls:
> 1) xc_physdev_map_pirq (this is also called from libxl)
> 2) xc_domain_bind_pt_pci_irq
> 
> 
> In this case IRQ != GSI (IRQ == 112, GSI == 28). Sysfs returns the IRQ
> in Linux (112), but actually xc_physdev_map_pirq expects the GSI, not
> the IRQ. If you look at the implementation of xc_physdev_map_pirq,
> you'll the type is "MAP_PIRQ_TYPE_GSI" and also see the check in Xen
> xen/arch/x86/irq.c:allocate_and_map_gsi_pirq:
> 
>     if ( index < 0 || index >= nr_irqs_gsi )
>     {
>         dprintk(XENLOG_G_ERR, "dom%d: map invalid irq %d\n", d->domain_id,
>                 index);
>         return -EINVAL;
>     }
> 
> nr_irqs_gsi < 112, and the check will fail.
> 
> So we need to pass the GSI to xc_physdev_map_pirq. To do that, we need
> to discover the GSI number corresponding to the IRQ number.

That's one possible approach. Another could be (making a lot of assumptions)
that a PVH Dom0 would pass in the IRQ it knows for this interrupt and Xen
then translates that to GSI, knowing that PVH doesn't have (host) GSIs
exposed to it.

Jan