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

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Monday, November 2, 2020 9:22 PM
> 
> 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.

Hypercall is VMM specific, while IMS cap provides a VMM-agnostic
interface so any guest driver (if following the spec) can seamlessly
work on all hypervisors.

Thanks
Kevin