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    SubjectRE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1 format to userspace
    Date
    > From: Tian, Kevin <kevin.tian@intel.com>
    > Sent: Wednesday, April 1, 2020 3:56 PM
    > To: Liu, Yi L <yi.l.liu@intel.com>; alex.williamson@redhat.com;
    > Subject: RE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1 format to
    > userspace
    >
    > > From: Liu, Yi L <yi.l.liu@intel.com>
    > > Sent: Wednesday, April 1, 2020 3:38 PM
    > >
    > > > From: Tian, Kevin <kevin.tian@intel.com>
    > > > Sent: Monday, March 30, 2020 7:49 PM
    > > > To: Liu, Yi L <yi.l.liu@intel.com>; alex.williamson@redhat.com;
    > > > Subject: RE: [PATCH v1 5/8] vfio/type1: Report 1st-level/stage-1
    > > > format to userspace
    > > >
    > > > > From: Liu, Yi L <yi.l.liu@intel.com>
    > > > > Sent: Sunday, March 22, 2020 8:32 PM
    > > > >
    > > > > From: Liu Yi L <yi.l.liu@intel.com>
    > > > >
    > > > > 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@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>
    > > > > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
    > > > > ---
    > > > > 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

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