RE: [PATCH v2 14/15] vfio: Document dual stage control

["Liu, Yi L" <yi.l.liu@intel.com>]

> From: Stefan Hajnoczi <stefanha@gmail.com>
> Sent: Monday, June 15, 2020 5:41 PM
> On Thu, Jun 11, 2020 at 05:15:33AM -0700, Liu Yi L wrote:
>
> > From: Eric Auger <eric.auger@redhat.com>
> >
> > The VFIO API was enhanced to support nested stage control: a bunch of
> > new iotcls and usage guideline.
> >
> > Let's document the process to follow to set up nested mode.
> >
> > Cc: Kevin Tian <kevin.tian@intel.com>
> > CC: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Cc: Alex Williamson <alex.williamson@redhat.com>
> > Cc: Eric Auger <eric.auger@redhat.com>
> > Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Cc: Joerg Roedel <joro@8bytes.org>
> > Cc: Lu Baolu <baolu.lu@linux.intel.com>
> > Signed-off-by: Eric Auger <eric.auger@redhat.com>
> > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > ---
> > v1 -> v2:
> > *) new in v2, compared with Eric's original version, pasid table bind
> >    and fault reporting is removed as this series doesn't cover them.
> >    Original version from Eric.
> >    https://lkml.org/lkml/2020/3/20/700
> >
> >  Documentation/driver-api/vfio.rst | 64
> > +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 64 insertions(+)
> >
> > diff --git a/Documentation/driver-api/vfio.rst
> > b/Documentation/driver-api/vfio.rst
> > index f1a4d3c..06224bd 100644
> > --- a/Documentation/driver-api/vfio.rst
> > +++ b/Documentation/driver-api/vfio.rst
> > @@ -239,6 +239,70 @@ group and can access them as follows::
> >  	/* Gratuitous device reset and go... */
> >  	ioctl(device, VFIO_DEVICE_RESET);
> >
> > +IOMMU Dual Stage Control
> > +------------------------
> > +
> > +Some IOMMUs support 2 stages/levels of translation. Stage corresponds
> > +to the ARM terminology while level corresponds to Intel's VTD terminology.
> > +In the following text we use either without distinction.
> > +
> > +This is useful when the guest is exposed with a virtual IOMMU and
> > +some devices are assigned to the guest through VFIO. Then the guest
> > +OS can use stage 1 (GIOVA -> GPA or GVA->GPA), while the hypervisor
> > +uses stage 2 for VM isolation (GPA -> HPA).
> > +
> > +Under dual stage translation, the guest gets ownership of the stage 1
> > +page tables and also owns stage 1 configuration structures. The
> > +hypervisor owns the root configuration structure (for security
> > +reason), including stage 2 configuration. This works as long
> > +configuration structures and page table
> 
> s/as long configuration/as long as configuration/

got it.

> 
> > +format are compatible between the virtual IOMMU and the physical IOMMU.
> 
> s/format/formats/

I see.

> > +
> > +Assuming the HW supports it, this nested mode is selected by choosing
> > +the VFIO_TYPE1_NESTING_IOMMU type through:
> > +
> > +    ioctl(container, VFIO_SET_IOMMU, VFIO_TYPE1_NESTING_IOMMU);
> > +
> > +This forces the hypervisor to use the stage 2, leaving stage 1
> > +available for guest usage. The guest stage 1 format depends on IOMMU
> > +vendor, and it is the same with the nesting configuration method.
> > +User space should check the format and configuration method after
> > +setting nesting type by
> > +using:
> > +
> > +    ioctl(container->fd, VFIO_IOMMU_GET_INFO, &nesting_info);
> > +
> > +Details can be found in Documentation/userspace-api/iommu.rst. For
> > +Intel VT-d, each stage 1 page table is bound to host by:
> > +
> > +    nesting_op->flags = VFIO_IOMMU_NESTING_OP_BIND_PGTBL;
> > +    memcpy(&nesting_op->data, &bind_data, sizeof(bind_data));
> > +    ioctl(container->fd, VFIO_IOMMU_NESTING_OP, nesting_op);
> > +
> > +As mentioned above, guest OS may use stage 1 for GIOVA->GPA or GVA->GPA.
> > +GVA->GPA page tables are available when PASID (Process Address Space
> > +GVA->ID)
> > +is exposed to guest. e.g. guest with PASID-capable devices assigned.
> > +For such page table binding, the bind_data should include PASID info,
> > +which is allocated by guest itself or by host. This depends on
> > +hardware vendor e.g. Intel VT-d requires to allocate PASID from host.
> > +This requirement is available by VFIO_IOMMU_GET_INFO. User space
> > +could allocate PASID from host by:
> > +
> > +    req.flags = VFIO_IOMMU_ALLOC_PASID;
> > +    ioctl(container, VFIO_IOMMU_PASID_REQUEST, &req);
> 
> It is not clear how the userspace application determines whether PASIDs must be
> allocated from the host via VFIO_IOMMU_PASID_REQUEST or if the guest itself can
> allocate PASIDs. The text mentions VFIO_IOMMU_GET_INFO but what exactly
> should the userspace application check?

For VT-d, spec 3.0 introduced Virtual Cmd interface for PASID allocation,
guest request PASID from host if it detects the interface. Application
should check the IOMMU_NESTING_FEAT_SYSWIDE_PASID setting in the below
info reported by VFIO_IOMMU_GET_INFO. And virtual VT-d should not report
SVA related capabilities to guest if  SYSWIDE_PASID is not supported by
kernel.

+struct iommu_nesting_info {
+	__u32	size;
+	__u32	format;
+	__u32	features;
+#define IOMMU_NESTING_FEAT_SYSWIDE_PASID	(1 << 0)
+#define IOMMU_NESTING_FEAT_BIND_PGTBL		(1 << 1)
+#define IOMMU_NESTING_FEAT_CACHE_INVLD		(1 << 2)
+	__u32	flags;
+	__u8	data[];
+};
https://lore.kernel.org/linux-iommu/1591877734-66527-3-git-send-email-yi.l.liu@intel.com/

Regards,
Yi Liu