Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

[Jason Gunthorpe]

On Fri, Oct 30, 2020 at 03:49:22PM -0700, Dave Jiang wrote:
> 
> 
> On 10/30/2020 3:45 PM, Jason Gunthorpe wrote:
> > On Fri, Oct 30, 2020 at 02:20:03PM -0700, Dave Jiang wrote:
> > > So the intel-iommu driver checks for the SIOV cap. And the idxd driver
> > > checks for SIOV and IMS cap. There will be other upcoming drivers that will
> > > check for such cap too. It is Intel vendor specific right now, but SIOV is
> > > public and other vendors may implement to the spec. Is there a good place to
> > > put the common capability check for that?
> > 
> > I'm still really unhappy with these SIOV caps. It was explained this
> > is just a hack to make up for pci_ims_array_create_msi_irq_domain()
> > succeeding in VM cases when it doesn't actually work.
> > 
> > Someday this is likely to get fixed, so tying platform behavior to PCI
> > caps is completely wrong.
> > 
> > This needs to be solved in the platform code,
> > pci_ims_array_create_msi_irq_domain() should not succeed in these
> > cases.
> 
> That sounds reasonable. Are you asking that the IMS cap check should gate
> the success/failure of pci_ims_array_create_msi_irq_domain() rather than the
> driver?

There shouldn't be an IMS cap at all

As I understand, the problem here is the only way to establish new
VT-d IRQ routing is by trapping and emulating MSI/MSI-X related
activities and triggering routing of the vectors into the guest.

There is a missing hypercall to allow the guest to do this on its own,
presumably it will someday be fixed so IMS can work in guests.

Until the hypercall is added pci_ims_array_create_msi_irq_domain()
should simply fail in guests. No PCI cap check required.

Jason