RE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1 format to userspace

[Liu, Yi L]

> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Sent: Wednesday, April 1, 2020 3:56 PM
> To: Liu, Yi L <yi.l.liu@xxxxxxxxx>; alex.williamson@xxxxxxxxxx;
> Subject: RE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1 format to
> userspace
> 
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Wednesday, April 1, 2020 3:38 PM
> >
> >  > From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> > > Sent: Monday, March 30, 2020 7:49 PM
> > > To: Liu, Yi L <yi.l.liu@xxxxxxxxx>; alex.williamson@xxxxxxxxxx;
> > > Subject: RE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1
> > > format to userspace
> > >
> > > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > > Sent: Sunday, March 22, 2020 8:32 PM
> > > >
> > > > From: Liu Yi L <yi.l.liu@xxxxxxxxx>
> > > >
> > > > VFIO exposes IOMMU nesting translation (a.k.a dual stage
> > > > translation) capability to userspace. Thus applications like QEMU
> > > > could support vIOMMU with hardware's nesting translation
> > > > capability for pass-through devices. Before setting up nesting
> > > > translation for pass-through devices, QEMU and other applications
> > > > need to learn the supported
> > > > 1st-lvl/stage-1 translation structure format like page table format.
> > > >
> > > > Take vSVA (virtual Shared Virtual Addressing) as an example, to
> > > > support vSVA for pass-through devices, QEMU setup nesting
> > > > translation for pass- through devices. The guest page table are
> > > > configured to host as 1st-lvl/
> > > > stage-1 page table. Therefore, guest format should be compatible
> > > > with host side.
> > > >
> > > > This patch reports the supported 1st-lvl/stage-1 page table format
> > > > on the current platform to userspace. QEMU and other alike
> > > > applications should use this format info when trying to setup
> > > > IOMMU nesting translation on host IOMMU.
> > > >
> > > > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > > > CC: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > Cc: Eric Auger <eric.auger@xxxxxxxxxx>
> > > > Cc: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> > > > Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> > > > ---
> > > >  drivers/vfio/vfio_iommu_type1.c | 56
> > > > +++++++++++++++++++++++++++++++++++++++++
> > > >  include/uapi/linux/vfio.h       |  1 +
> > > >  2 files changed, 57 insertions(+)
> > > >
> > > > diff --git a/drivers/vfio/vfio_iommu_type1.c
> > > > b/drivers/vfio/vfio_iommu_type1.c index 9aa2a67..82a9e0b 100644
> > > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > @@ -2234,11 +2234,66 @@ static int
> > vfio_iommu_type1_pasid_free(struct
> > > > vfio_iommu *iommu,
> > > >  	return ret;
> > > >  }
> > > >
> > > > +static int vfio_iommu_get_stage1_format(struct vfio_iommu *iommu,
> > > > +					 u32 *stage1_format)
> > > > +{
> > > > +	struct vfio_domain *domain;
> > > > +	u32 format = 0, tmp_format = 0;
> > > > +	int ret;
> > > > +
> > > > +	mutex_lock(&iommu->lock);
> > > > +	if (list_empty(&iommu->domain_list)) {
> > > > +		mutex_unlock(&iommu->lock);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	list_for_each_entry(domain, &iommu->domain_list, next) {
> > > > +		if (iommu_domain_get_attr(domain->domain,
> > > > +			DOMAIN_ATTR_PASID_FORMAT, &format)) {
> > > > +			ret = -EINVAL;
> > > > +			format = 0;
> > > > +			goto out_unlock;
> > > > +		}
> > > > +		/*
> > > > +		 * format is always non-zero (the first format is
> > > > +		 * IOMMU_PASID_FORMAT_INTEL_VTD which is 1). For
> > > > +		 * the reason of potential different backed IOMMU
> > > > +		 * formats, here we expect to have identical formats
> > > > +		 * in the domain list, no mixed formats support.
> > > > +		 * return -EINVAL to fail the attempt of setup
> > > > +		 * VFIO_TYPE1_NESTING_IOMMU if non-identical formats
> > > > +		 * are detected.
> > > > +		 */
> > > > +		if (tmp_format && tmp_format != format) {
> > > > +			ret = -EINVAL;
> > > > +			format = 0;
> > > > +			goto out_unlock;
> > > > +		}
> > > > +
> > > > +		tmp_format = format;
> > > > +	}
> > >
> > > this path is invoked only in VFIO_IOMMU_GET_INFO path. If we don't
> > > want
> > to
> > > assume the status quo that one container holds only one device w/
> > vIOMMU
> > > (the prerequisite for vSVA), looks we also need check the format
> > > compatibility when attaching a new group to this container?
> >
> > right. if attaching to a nesting type container (vfio_iommu.nesting
> > bit indicates it), it should check if it is compabile with prior
> > domains in the domain list. But if it is the first one attached to
> > this container, it's fine. is it good?
> 
> yes, but my point is whether we should check the format compatibility
> in the attach path...

I guess so. Assume a device has been attached to a container, and
userspace has fetched the nesting cap info. e.g. QEMU will have a
per-container structure to store the nesting info. And then attach
another device from a separate group, if its backend iommu supports
different formats, then it will be a problem. If userspace reads the
nesting cap info again, it will get a different value. It may affect
the prior attched device. If userspace doesn't refresh the nesting
info by re-fetch, then the newly added device may use a format which
its backend iommu doesn't support.

Although, the vendor specific iommu driver should ensure all devices
are backed by iommu units w/ same capability (e.g. format). But it
would better to have a check in vfio side all the same. how about your
opinion so far?:-)

Regards,
Yi Liu