Re: [RFC 3/3] vfio/pci: Expose PCIe PASID capability to userspace

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

On Wed, 27 Sep 2023 08:07:54 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Tuesday, September 26, 2023 5:31 PM
> > 
> > This exposes PCIe PASID capability to userspace and where to emulate this
> > capability if wants to further expose it to VM.
> > 
> > And this only exposes PASID capability for devices which has PCIe PASID
> > extended struture in its configuration space. While for VFs, userspace
> > is still unable to see this capability as SR-IOV spec forbides VF to
> > implement PASID capability extended structure. It is a TODO in future.
> > Related discussion can be found in below links:
> > 
> > https://lore.kernel.org/kvm/20200407095801.648b1371@w520.home/
> > https://lore.kernel.org/kvm/BL1PR11MB5271A60035EF591A5BE8AC878C01A
> > @BL1PR11MB5271.namprd11.prod.outlook.com/
> >   
> 
> Yes, we need a decision for VF case.
> 
> If the consensus is to continue exposing the PASID capability in vfio-pci
> config space by developing a kernel quirk mechanism to find offset for
> VF, then this patch for PF is orthogonal to that VF work and can go as it is.
> 
> But if the decision is to have a device feature for the user to enumerate
> the vPASID capability and let the VMM take care of finding the vPASID
> cap offset, then better we start doing that for PF too since it's not good
> to have two enumeration interfaces for PF/VF respectively.

Note also that QEMU implements a lazy algorithm for exposing
capabilities, the default is to expose them, so we need to consider
existing VMs seeing a new read-only PASID capability on an assigned PF.

That might support an alternate means to expose the capability.

> My preference is via device feature given Qemu already includes lots of
> quirks for vfio-pci devices. Another reason is that when supporting vPASID
> with SIOV there are some arch constraints which the driver needs to
> report to the user to follow (e.g.  don't assign ENQCMD-capable sibling
> vdev's to a same guest, etc.).

?!

> A device feature interface can better
> encapsulate everything related to vPASID in one place.

Sorry if I don't remember, have you posted a proposal for the device
feature interface?  Thanks,

Alex