RE: vPASID capability for VF

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Wednesday, May 10, 2023 6:44 AM
> 
> On Tue, May 09, 2023 at 08:34:53AM +0000, Tian, Kevin wrote:
> > According to PCIe spec (7.8.9 PASID Extended Capability Structure):
> >
> >   The PASID configuration of the single non-VF Function representing
> >   the device is also used by all VFs in the device. A PF is permitted
> >   to implement the PASID capability, but VFs must not implement it.
> >
> > To enable PASID on VF then one open is where to locate the PASID
> > capability in VF's vconfig space. vfio-pci doesn't know which offset
> > may contain VF specific config registers. Finding such offset must
> > come from a device specific knowledge.
> 
> Why? Can't vfio probe the cap tree and just find a gap to insert a new
> cap? We already mangle the cap list, I'm not sure I see what
> the problem is?
> 

PCI config space includes not only caps, but also device specific
defined fields. e.g. Intel IGD defines offset 0xfc as a pointer to
OpRegion. I'm sure Alex can give many other examples.

So it's easy to find the gap between caps, but not easy to know
whether that gap is actually free to use.