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

Tian, Kevin
2021-04-02T10:01:49Z

> From: Tian, Kevin
> Sent: Friday, April 2, 2021 3:58 PM
>
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > 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@nvidia.com>
> > > > 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@nvidia.com>
> > > > > > 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). Although we have some
> idea relaxing this restriction in the non-identity scheme, it requires
> more thinking given how the vSVA uAPI is being refactored.
>
> In both cases the virtual VT-d will report a virtual capability to the guest,
> indicating that the guest must request PASID through a vcmd register
> instead of creating its own namespace. The vIOMMU returns a vPASID
> to the guest upon request. The vPASID could be directly mapped to a
> pPASID or allocated from a new namespace based on user configuration.
>
> 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.
>

Per your comments in other threads I suppose this requirement should
be implemented in VFIO_ALLOW_PASID command instead of going
through /dev/ioasid which only needs to know pPASID and its pgtable
management.

Thanks
Kevin