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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>, "Liu, Yi L" <yi.l.liu@intel.com>
CC:     Jacob Pan <jacob.jun.pan@linux.intel.com>,
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        "Jean-Philippe Brucker" <jean-philippe@linaro.com>,
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        "Jonathan Corbet" <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs
Thread-Topic: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
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Date:   Fri, 2 Apr 2021 07:58:02 +0000
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> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, April 1, 2021 9:47 PM
>=20
> 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 wit=
hout
> > > > 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, an=
d
> > > you can't really use a vPASID.
> >
> > This is a diagram shows the vSVA setup.
>=20
> I'm not talking only about vSVA. Generic PASID support with arbitary
> mappings.
>=20
> And how do you deal with the vPASID vs pPASID issue if the system has
> a mix of physical devices and mdevs?
>=20

We plan to support two schemes. One is vPASID identity-mapped to
pPASID then the mixed scenario just works, with the limitation of=20
lacking of live migration support. The other is non-identity-mapped=20
scheme, where live migration is supported but physical devices and=20
mdevs should not be mixed in one VM if both expose SVA capability=20
(requires some filtering check in Qemu). Although we have some=20
idea relaxing this restriction in the non-identity scheme, it requires=20
more thinking given how the vSVA uAPI is being refactored.

In both cases the virtual VT-d will report a virtual capability to the gues=
t,
indicating that the guest must request PASID through a vcmd register
instead of creating its own namespace. The vIOMMU returns a vPASID=20
to the guest upon request. The vPASID could be directly mapped to a=20
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.

Thanks
Kevin