RE: [PATCH v2 02/15] iommu: Report domain nesting info

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

Hi Kevin,

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Monday, June 15, 2020 9:23 AM
> 
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Friday, June 12, 2020 5:05 PM
> >
> > Hi Alex,
> >
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Friday, June 12, 2020 3:30 AM
> > >
> > > On Thu, 11 Jun 2020 05:15:21 -0700
> > > Liu Yi L <yi.l.liu@intel.com> wrote:
> > >
> > > > IOMMUs that support nesting translation needs report the
> > > > capability info to userspace, e.g. the format of first level/stage paging
> structures.
> > > >
> > > > 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: Liu Yi L <yi.l.liu@intel.com>
> > > > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > > ---
> > > > @Jean, Eric: as nesting was introduced for ARM, but looks like no
> > > > actual user of it. right? So I'm wondering if we can reuse
> > > > DOMAIN_ATTR_NESTING to retrieve nesting info? how about your
> > opinions?
> > > >
> > > >  include/linux/iommu.h      |  1 +
> > > >  include/uapi/linux/iommu.h | 34
> > ++++++++++++++++++++++++++++++++++
> > > >  2 files changed, 35 insertions(+)
> > > >
> > > > diff --git a/include/linux/iommu.h b/include/linux/iommu.h index
> > > > 78a26ae..f6e4b49 100644
> > > > --- a/include/linux/iommu.h
> > > > +++ b/include/linux/iommu.h
> > > > @@ -126,6 +126,7 @@ enum iommu_attr {
> > > >  	DOMAIN_ATTR_FSL_PAMUV1,
> > > >  	DOMAIN_ATTR_NESTING,	/* two stages of translation */
> > > >  	DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE,
> > > > +	DOMAIN_ATTR_NESTING_INFO,
> > > >  	DOMAIN_ATTR_MAX,
> > > >  };
> > > >
> > > > diff --git a/include/uapi/linux/iommu.h
> > > > b/include/uapi/linux/iommu.h index 303f148..02eac73 100644
> > > > --- a/include/uapi/linux/iommu.h
> > > > +++ b/include/uapi/linux/iommu.h
> > > > @@ -332,4 +332,38 @@ struct iommu_gpasid_bind_data {
> > > >  	};
> > > >  };
> > > >
> > > > +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[];
> > > > +};
> > > > +
> > > > +/*
> > > > + * @flags:	VT-d specific flags. Currently reserved for future
> > > > + *		extension.
> > > > + * @addr_width:	The output addr width of first level/stage
> translation
> > > > + * @pasid_bits:	Maximum supported PASID bits, 0 represents no
> > PASID
> > > > + *		support.
> > > > + * @cap_reg:	Describe basic capabilities as defined in VT-d
> > capability
> > > > + *		register.
> > > > + * @cap_mask:	Mark valid capability bits in @cap_reg.
> > > > + * @ecap_reg:	Describe the extended capabilities as defined in VT-d
> > > > + *		extended capability register.
> > > > + * @ecap_mask:	Mark the valid capability bits in @ecap_reg.
> > >
> > > Please explain this a little further, why do we need to tell
> > > userspace about cap/ecap register bits that aren't valid through this interface?
> > > Thanks,
> >
> > we only want to tell userspace about the bits marked in the cap/ecap_mask.
> > cap/ecap_mask is kind of white-list of the cap/ecap register.
> > userspace should only care about the bits in the white-list, for other
> > bits, it should ignore.
> >
> > Regards,
> > Yi Liu
> 
> For invalid bits if kernel just clears them then do we still need additional mask bits
> to explicitly mark them out? I guess this might be the point that Alex asked...

For invalid bits, kernel will clear them. But I think the mask bits is
still necessary. The mask bits tells user space the bits related to
nesting. Without it, user space may have no idea about it.

Maybe talk about QEMU usage of the cap/ecap bits would help. QEMU vIOMMU
decides cap/ecap bits according to QEMU cmdline. But not all of them are
compatible with hardware support. Especially, vIOMMU built on nesting.
So needs to sync the cap/ecap bits with host side. Based on the mask
bits, QEMU can compare the cap/ecap bits configured by QEMU cmdline with
the cap/ecap bits reported by this interface. This comparation is limited
to the nesting related bits in cap/ecap, the other bits are not included
and can use the configuration by QEMU cmdline.

The link below show the current Intel vIOMMU usage on the cap/ecap bits.
For each assigned device, vIOMMU will compare the nesting related bits in
cap/ecap and mask out the bits which hardware doesn't support. After the
machine is intilized, the vIOMMU cap/ecap bits are determined. If user
hot-plug devices to VM, vIOMMU will fail it if the hardware cap/ecap bits
behind hot-plug device are not compatible with determined vIOMMU cap/ecap
bits.

https://www.spinics.net/lists/kvm/msg218294.html

Regards,
Yi Liu

> >
> > > Alex
> > >
> > >
> > > > + */
> > > > +struct iommu_nesting_info_vtd {
> > > > +	__u32	flags;
> > > > +	__u16	addr_width;
> > > > +	__u16	pasid_bits;
> > > > +	__u64	cap_reg;
> > > > +	__u64	cap_mask;
> > > > +	__u64	ecap_reg;
> > > > +	__u64	ecap_mask;
> > > > +};
> > > > +
> > > >  #endif /* _UAPI_IOMMU_H */

_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu