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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Mar 16, 2023 at 09:55:03AM +0100, Jan Beulich wrote:
> 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@amd.com>
> >>>
> >>> Use new xc_physdev_gsi_from_irq to get the GSI number
> >>>
> >>> Signed-off-by: Chen Jiqian <Jiqian.Chen@amd.com>
> >>> Signed-off-by: Huang Rui <ray.huang@amd.com>
> >>> ---
> >>>  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.

I don't think Xen can translate a Linux IRQ to a GSI, as that's a
Linux abstraction Xen has no part in.

The GSIs exposed to a PVH dom0 are the native (host) ones, as we
create an emulated IO-APIC topology that mimics the physical one.

Question here is why Linux ends up with a IRQ != GSI, as it's my
understanding on Linux GSIs will always be identity mapped to IRQs, and
the IRQ space up to the last possible GSI is explicitly reserved for
this purpose.

Thanks, Roger.