Re: [PATCH v3 02/14] iommu: Report domain nesting info

From: Stefan Hajnoczi
Date: Fri Jul 03 2020 - 05:59:09 EST


On Tue, Jun 30, 2020 at 02:00:49AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Monday, June 29, 2020 8:23 PM
> >
> > Hi Stefan,
> >
> > > From: Stefan Hajnoczi <stefanha@xxxxxxxxx>
> > > 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 |
> > > > + *
> > >
> > +===============+===============================================
> > ====
> > > =+
> > > > + * | SYSWIDE_PASID | Kernel manages PASID in system wide, PASIDs
> > used |
> > > > + * | | in the system should be allocated by host kernel |
> > > > + * +---------------+----------------------------------------------------+
> > > > + * | BIND_PGTBL | bind page tables to host PASID, the PASID could |
> > > > + * | | either be a host PASID passed in bind request or |
> > > > + * | | default PASIDs (e.g. default PASID of aux-domain) |
> > > > + * +---------------+----------------------------------------------------+
> > > > + * | 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.
> >
> > 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?
> >
>
> So far this assumption is correct but it may not be true when thinking forward.
> 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. From
> 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
> 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
> bind the page table to associated PASID (either the one specified in bind
> 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.
> --

Mentioning the API to allocate/free PASIDs and VFIO_IOMMU_NESTING_OP has
made this clearer. This lets someone reading the documentation know
where to look for further information on using these features.

Thank you!

Stefan

Attachment: signature.asc
Description: PGP signature