Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Jason Gunthorpe]

On Tue, Apr 06, 2021 at 01:02:05AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Tuesday, April 6, 2021 7:40 AM
> > 
> > On Fri, Apr 02, 2021 at 07:58:02AM +0000, Tian, Kevin wrote:
> > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > Sent: Thursday, April 1, 2021 9:47 PM
> > > >
> > > > On Thu, Apr 01, 2021 at 01:43:36PM +0000, Liu, Yi L wrote:
> > > > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > > > Sent: Thursday, April 1, 2021 9:16 PM
> > > > > >
> > > > > > On Thu, Apr 01, 2021 at 01:10:48PM +0000, Liu, Yi L wrote:
> > > > > > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > > > > > Sent: Thursday, April 1, 2021 7:47 PM
> > > > > > > [...]
> > > > > > > > I'm worried Intel views the only use of PASID in a guest is with
> > > > > > > > ENQCMD, but that is not consistent with the industry. We need to
> > see
> > > > > > > > normal nested PASID support with assigned PCI VFs.
> > > > > > >
> > > > > > > I'm not quire flow here. Intel also allows PASID usage in guest
> > without
> > > > > > > ENQCMD. e.g. Passthru a PF to guest, and use PASID on it without
> > > > > > ENQCMD.
> > > > > >
> > > > > > Then you need all the parts, the hypervisor calls from the vIOMMU,
> > and
> > > > > > you can't really use a vPASID.
> > > > >
> > > > > This is a diagram shows the vSVA setup.
> > > >
> > > > I'm not talking only about vSVA. Generic PASID support with arbitary
> > > > mappings.
> > > >
> > > > And how do you deal with the vPASID vs pPASID issue if the system has
> > > > a mix of physical devices and mdevs?
> > > >
> > >
> > > We plan to support two schemes. One is vPASID identity-mapped to
> > > pPASID then the mixed scenario just works, with the limitation of
> > > lacking of live migration support. The other is non-identity-mapped
> > > scheme, where live migration is supported but physical devices and
> > > mdevs should not be mixed in one VM if both expose SVA capability
> > > (requires some filtering check in Qemu).
> > 
> > That just becomes "block vPASID support if any device that
> > doesn't use ENQCMD is plugged into the guest"
> 
> The limitation is only for physical device. and in reality it is not that
> bad. To support live migration with physical device we anyway need 
> additional work to migrate the device state (e.g. based on Max's work), 
> then it's not unreasonable to also mediate guest programming of 
> device specific PASID register to enable vPASID (need to translate in
> the whole VM lifespan but likely is not a hot path).

IMHO that is pretty unreasonable.. More likely we end up with vPASID
tables in each migratable device like KVM has.

> > Which needs a special VFIO capability of some kind so qemu knows to
> > block it. This really needs to all be layed out together so someone
> > can understand it :(
> 
> Or could simply based on whether the VFIO device supports live migration.

You need to define affirmative caps that indicate that vPASID will be
supported by the VFIO device.

> > Why doesn't the siov cookbook explaining this stuff??
> > 
> > > We hope the /dev/ioasid can support both schemes, with the minimal
> > > requirement of allowing userspace to tag a vPASID to a pPASID and
> > > allowing mdev to translate vPASID into pPASID, i.e. not assuming that
> > > the guest will always use pPASID.
> > 
> > What I'm a unclear of is if /dev/ioasid even needs to care about
> > vPASID or if vPASID is just a hidden artifact of the KVM connection to
> > setup the translation table and the vIOMMU driver in qemu.
> 
> Not just for KVM. Also required by mdev, which needs to translate
> vPASID into pPASID when ENQCMD is not used.

Do we have any mdev's that will do this?

> should only care about the operations related to pPASID. VFIO could
> carry vPASID information to mdev.

It depends how common this is, I suppose

Jason