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

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Thursday, September 28, 2023 2:53 AM
> 
> On Wed, 27 Sep 2023 08:07:54 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > 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@xxxxxxxxx/
> > >
> 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.

Yep. that's also a valid point.

> 
> > 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.).
> 
> ?!

Sorry that I didn't plan to elaborate that tricky constraint before we show
the overall SIOV/vPASID implementation. Explaining it requires lots of
context and here just want to mention the potential requirement in case
we need more proofs to go this direction. 😊

> 
> > 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,
> 

Not yet. Will do in next version.