Re: [PATCH v6 4/5] [FUTURE] xen/arm: enable vPCI for domUs

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 1 Dec 2023, Roger Pau Monné wrote:
> On Mon, Nov 13, 2023 at 05:21:13PM -0500, Stewart Hildebrand wrote:
> > @@ -1618,6 +1630,14 @@ int iommu_do_pci_domctl(
> >          bus = PCI_BUS(machine_sbdf);
> >          devfn = PCI_DEVFN(machine_sbdf);
> >  
> > +        if ( needs_vpci(d) && !has_vpci(d) )
> > +        {
> > +            printk(XENLOG_G_WARNING "Cannot assign %pp to %pd: vPCI 
> > support not enabled\n",
> > +                   &PCI_SBDF(seg, bus, devfn), d);
> > +            ret = -EPERM;
> > +            break;
> 
> I think this is likely too restrictive going forward.  The current
> approach is indeed to enable vPCI on a per-domain basis because that's
> how PVH dom0 uses it, due to being unable to use ioreq servers.
> 
> If we start to expose vPCI suport to guests the interface should be on
> a per-device basis, so that vPCI could be enabled for some devices,
> while others could still be handled by ioreq servers.
> 
> We might want to add a new flag to xen_domctl_assign_device (used by
> XEN_DOMCTL_assign_device) in order to signal whether the device will
> use vPCI.

Actually I don't think this is a good idea. I am all for flexibility but
supporting multiple different configurations comes at an extra cost for
both maintainers and contributors. I think we should try to reduce the
amount of configurations we support rather than increasing them
(especially on x86 where we have PV, PVH, HVM).

I don't think we should enable IOREQ servers to handle PCI passthrough
for PVH guests and/or guests with vPCI. If the domain has vPCI, PCI
Passthrough can be handled by vPCI just fine. I think this should be a
good anti-feature to have (a goal to explicitly not add this feature) to
reduce complexity. Unless you see a specific usecase to add support for
it?