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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     "Liu, Yi L" <yi.l.liu@intel.com>,
        Stefan Hajnoczi <stefanha@gmail.com>
CC:     "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
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        "Tian, Jun J" <jun.j.tian@intel.com>,
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        "jean-philippe@linaro.org" <jean-philippe@linaro.org>,
        "peterx@redhat.com" <peterx@redhat.com>,
        "Wu, Hao" <hao.wu@intel.com>,
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Subject: RE: [PATCH v3 02/14] iommu: Report domain nesting info
Thread-Topic: [PATCH v3 02/14] iommu: Report domain nesting info
Thread-Index: AQHWSgRQYPLf19tsF0i4Fom/j2zC7qjvWjsAgAAx1ACAANlZcA==
Date:   Tue, 30 Jun 2020 02:00:49 +0000
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> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Monday, June 29, 2020 8:23 PM
>=20
> Hi Stefan,
>=20
> > From: Stefan Hajnoczi <stefanha@gmail.com>
> > Sent: Monday, June 29, 2020 5:25 PM
> >
> > On Wed, Jun 24, 2020 at 01:55:15AM -0700, Liu Yi L wrote:
> > > +/*
> > > + * struct iommu_nesting_info - Information for nesting-capable IOMMU=
.
> > > + *				user space should check it before using
> > > + *				nesting capability.
> > > + *
> > > + * @size:	size of the whole structure
> > > + * @format:	PASID table entry format, the same definition with
> > > + *		@format of struct iommu_gpasid_bind_data.
> > > + * @features:	supported nesting features.
> > > + * @flags:	currently reserved for future extension.
> > > + * @data:	vendor specific cap info.
> > > + *
> > > + * +---------------+------------------------------------------------=
----+
> > > + * | feature       |  Notes                                         =
    |
> > > + *
> >
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D+=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> =3D=3D=3D=3D
> > =3D+
> > > + * | SYSWIDE_PASID |  Kernel manages PASID in system wide, PASIDs
> used  |
> > > + * |               |  in the system should be allocated by host kern=
el  |
> > > + * +---------------+------------------------------------------------=
----+
> > > + * | BIND_PGTBL    |  bind page tables to host PASID, the PASID coul=
d   |
> > > + * |               |  either be a host PASID passed in bind request =
or  |
> > > + * |               |  default PASIDs (e.g. default PASID of aux-doma=
in) |
> > > + * +---------------+------------------------------------------------=
----+
> > > + * | CACHE_INVLD   |  mandatory feature for nesting capable IOMMU
> |
> > > + * +---------------+------------------------------------------------=
----+
> >
> > This feature description is vague about what CACHE_INVLD does and how
> to
> > use it. If I understand correctly, the presence of this feature means
> > that VFIO_IOMMU_NESTING_OP_CACHE_INVLD must be used?
> >
> > The same kind of clarification could be done for SYSWIDE_PASID and
> > BIND_PGTBL too.
>=20
> For SYSWIDE_PASID and BIND_PGTBL, yes, presence of the feature bit
> means must use. So the two are requirements to user space if it wants
> to setup nesting. While for CACHE_INVLD, it's kind of availability
> here. How about removing CACHE_INVLD as presence of BIND_PGTBL should
> indicates support of CACHE_INVLD?
>=20

So far this assumption is correct but it may not be true when thinking forw=
ard.
For example, a vendor might find a way to allow the owner of 1st-level page
table to directly invalidate cache w/o going through host IOMMU driver. Fro=
m
this angle I feel explicitly reporting this capability is more robust.

Regarding to the description, what about below?

--
SYSWIDE_PASID: PASIDs are managed in system-wide, instead of per device.
When a device is assigned to userspace or VM, proper uAPI (provided by=20
userspace driver framework, e.g. VFIO) must be used to allocate/free PASIDs
for the assigned device.

BIND_PGTBL: The owner of the first-level/stage-1 page table must explicitly=
=20
bind the page table to associated PASID (either the one specified in bind=20
request or the default PASID of the iommu domain), through VFIO_IOMMU
_NESTING_OP

CACHE_INVLD: The owner of the first-level/stage-1 page table must
explicitly invalidate the IOMMU cache through VFIO_IOMMU_NESTING_OP,
according to vendor-specific requirement when changing the page table.
--

Thanks
Kevin