Re: [Qemu-devel] [PATCH for-2.11] vfio: Fix vfio-kvm group registration

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 6 Dec 2017 10:44:43 +0800
"Liu, Yi L" <yi.l.liu@linux.intel.com> wrote:

> On Tue, Dec 05, 2017 at 02:09:07PM -0700, Alex Williamson wrote:
> > Commit 8c37faa475f3 ("vfio-pci, ppc64/spapr: Reorder group-to-container
> > attaching") moved registration of groups with the vfio-kvm device from
> > vfio_get_group() to vfio_connect_container(), but it missed the case
> > where a group is attached to an existing container and takes an early
> > exit.  Perhaps this is a less common case on ppc64/spapr, but on x86
> > (without viommu) all groups are connected to the same container and
> > thus only the first group gets registered with the vfio-kvm device.
> > This becomes a problem if we then hot-unplug the devices associated
> > with that first group and we end up with KVM being misinformed about
> > any vfio connections that might remain.  Fix by including the call to
> > vfio_kvm_device_add_group() in this early exit path.
> > 
> > Fixes: 8c37faa475f3 ("vfio-pci, ppc64/spapr: Reorder group-to-container attaching")
> > Cc: qemu-stable@nongnu.org # qemu-2.10+
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> > 
> > This bug also existed in QEMU 2.10, but I think the fix is sufficiently
> > obvious (famous last words) to propose for 2.11 at this late date.  If
> > the first group is hot unplugged then KVM may revert to code emulation
> > that assumes no non-coherent DMA is present on some systems.  Also for
> > KVMGT, if the vGPU is not the first device registered, then the
> > notifier to enable linkages to KVM would not be called.  Please review.
> > Thanks,  
> 
> Alex, for x86, I suppose it doesn't exist in the case which viommu is exposed
> to guest?

With viommu, I believe each group would be in its own AddressSpace and
therefore get a separate container, so I don't think it'd be an issue.
It's only subsequent groups added to the same container which are
missed.  Thanks,

Alex

> >  hw/vfio/common.c |    1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> > index 7b2924c0ef19..7007878e345e 100644
> > --- a/hw/vfio/common.c
> > +++ b/hw/vfio/common.c
> > @@ -968,6 +968,7 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as,
> >          if (!ioctl(group->fd, VFIO_GROUP_SET_CONTAINER, &container->fd)) {
> >              group->container = container;
> >              QLIST_INSERT_HEAD(&container->group_list, group, container_next);
> > +            vfio_kvm_device_add_group(group);
> >              return 0;
> >          }
> >      }
> > 
> >