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

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

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.

> > Otherwise we could see about modifying the logic in PHYSDEVOP_map_pirq
> > so that the hardware domain can map IRQs to other domains without
> > having it mapped to itself first?
> 
> While this may be possible to arrange for, it still would feel wrong. How
> would you express the same in a disaggregated environment? I.e. how would
> you make sure a domain trying to grant permission is actually permitted to
> do so for the resource in question?

I've been looking again at the original issue, and I think I was
confused.  The issue is not that dom0 doesn't have permissions over
the GSIs (as we do grant those in dom0_setup_permissions()), but
rather that XEN_DOMCTL_irq_permission attempts to use
domain_pirq_to_irq() in order to get the IRQ from the PIRQ parameter.

I've always been a bit confused with the PIRQ GSI relation, but IIRC
GSIs are always identity mapped to PIRQs, and hence you could possibly
adjust XEN_DOMCTL_irq_permission to use irq_permit_access() to check
if the caller domain has permissions over the target PIRQ.

Thanks, Roger.