Re: [RFC XEN PATCH v4 4/5] domctl: Use gsi to grant/revoke irq permission

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jan 08, 2024 at 09:55:26AM +0100, Jan Beulich wrote:
> On 06.01.2024 02:08, Stefano Stabellini wrote:
> > On Fri, 5 Jan 2024, Jiqian Chen wrote:
> >> --- a/tools/libs/light/libxl_pci.c
> >> +++ b/tools/libs/light/libxl_pci.c
> >> @@ -1418,6 +1418,7 @@ static void pci_add_dm_done(libxl__egc *egc,
> >>      unsigned long long start, end, flags, size;
> >>      int irq, i;
> >>      int r;
> >> +    int gsi;
> >>      uint32_t flag = XEN_DOMCTL_DEV_RDM_RELAXED;
> >>      uint32_t domainid = domid;
> >>      bool isstubdom = libxl_is_stubdom(ctx, domid, &domainid);
> >> @@ -1486,6 +1487,7 @@ static void pci_add_dm_done(libxl__egc *egc,
> >>          goto out_no_irq;
> >>      }
> >>      if ((fscanf(f, "%u", &irq) == 1) && irq) {
> >> +        gsi = irq;
> > 
> > A question for Roger and Jan: are we always guaranteed that gsi == irq
> > (also in the PV case)?
> 
> Iirc for IO-APIC based IRQs that's always the case;

I think that's always the case on Linux, because it calls
PHYSDEVOP_map_pirq with index == pirq (see Linux
pci_xen_initial_domain()).  But other OSes could possibly make the
call with pirq == -1 and get a randomly allocated pirq for GSIs.

IOW: I don't think the pirq field in xen_domctl_irq_permission can be
altered like proposed here to switch from passing a pirq to a GSI.  A
new hypercall should be introduced that either is GSI specific, or
contains a type field in order to specify the namespace the field
targets.

Thanks, Roger.