Re: [PATCH 3/3] vfio: Report PASID capability via VFIO_DEVICE_FEATURE ioctl

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 11 Dec 2023 14:10:28 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Dec 11, 2023 at 11:03:45AM -0700, Alex Williamson wrote:
> > On Sun, 26 Nov 2023 22:39:09 -0800
> > Yi Liu <yi.l.liu@intel.com> wrote:

> > >    the PF). Creating a virtual PASID capability in vfio-pci config space needs
> > >    to find a hole to place it, but doing so may require device specific
> > >    knowledge to avoid potential conflict with device specific registers like
> > >    hiden bits in VF config space. It's simpler by moving this burden to the
> > >    VMM instead of maintaining a quirk system in the kernel.  
> > 
> > This feels a bit like an incomplete solution though and we might
> > already posses device specific knowledge in the form of a variant
> > driver.  Should this feature structure include a flag + field that
> > could serve to generically indicate to the VMM a location for
> > implementing the PASID capability?  The default core implementation
> > might fill this only for PFs where clearly an emualted PASID capability
> > can overlap the physical capability.  Thanks,  
> 
> In many ways I would perfer to solve this for good by having a way to
> learn a range of available config space - I liked the suggestion to
> use a DVSEC to mark empty space.

Yes, DVSEC is the most plausible option for the device itself to convey
unused config space, but that requires hardware adoption so presumably
we're going to need to fill the gaps with device specific code.  That
code might live in a variant driver or in the VMM.  If we have faith
that DVSEC is the way, it'd make sense for a variant driver to
implement a virtual DVSEC to work out the QEMU implementation and set a
precedent.

I mostly just want us to recognize that this feature structure also has
the possibility to fill this gap and we're consciously passing it over
and should maybe formally propose the DVSEC solution and reference it
in the commit log or comments here to provide a complete picture.
Thanks,

Alex