RE: [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

Liu, Yi L
2023-04-08T05:07:25Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Saturday, April 8, 2023 5:07 AM
>
> On Fri, 7 Apr 2023 15:47:10 +0000
> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Friday, April 7, 2023 11:14 PM
> > >
> > > On Fri, 7 Apr 2023 14:04:02 +0000
> > > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> > >
> > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > Sent: Friday, April 7, 2023 9:52 PM
> > > > >
> > > > > On Fri, 7 Apr 2023 13:24:25 +0000
> > > > > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> > > > >
> > > > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > > > Sent: Friday, April 7, 2023 8:04 PM
> > > > > > >
> > > > > > > > > > > @@ -791,7 +813,21 @@ static int vfio_pci_fill_devs(struct pci_dev
> > > *pdev,
> > > > > void
> > > > > > > > > *data)
> > > > > > > > > > > if (!iommu_group)
> > > > > > > > > > > return -EPERM; /* Cannot reset non-isolated devices
> */
> > > > > >
> > > > > > [1]
> > > > > >
> > > > > > > > > >
> > > > > > > > > > Hi Alex,
> > > > > > > > > >
> > > > > > > > > > Is disabling iommu a sane way to test vfio noiommu mode?
> > > > > > > > >
> > > > > > > > > Yes
> > > > > > > > >
> > > > > > > > > > I added intel_iommu=off to disable intel iommu and bind a device to
> vfio-
> > > pci.
> > > > > > > > > > I can see the /dev/vfio/noiommu-0 and /dev/vfio/devices/noiommu-
> vfio0.
> > > > > Bind
> > > > > > > > > > iommufd==-1 can succeed, but failed to get hot reset info due to the
> > > above
> > > > > > > > > > group check. Reason is that this happens to have some affected
> devices,
> > > and
> > > > > > > > > > these devices have no valid iommu_group (because they are not
> bound to
> > > > > vfio-
> > > > > > > pci
> > > > > > > > > > hence nobody allocates noiommu group for them). So when hot reset
> info
> > > > > loops
> > > > > > > > > > such devices, it failed with -EPERM. Is this expected?
> > > > > > > > >
> > > > > > > > > Hmm, I didn't recall that we put in such a limitation, but given the
> > > > > > > > > minimally intrusive approach to no-iommu and the fact that we never
> > > > > > > > > defined an invalid group ID to return to the user, it makes sense that
> > > > > > > > > we just blocked the ioctl for no-iommu use. I guess we can do the same
> > > > > > > > > for no-iommu cdev.
> > > > > > > >
> > > > > > > > I just realize a further issue related to this limitation. Remember that we
> > > > > > > > may finally compile out the vfio group infrastructure in the future. Say I
> > > > > > > > want to test noiommu, I may boot such a kernel with iommu disabled. I
> think
> > > > > > > > the _INFO ioctl would fail as there is no iommu_group. Does it mean we
> will
> > > > > > > > not support hot reset for noiommu in future if vfio group infrastructure is
> > > > > > > > compiled out?
> > > > > > >
> > > > > > > We're talking about IOMMU groups, IOMMU groups are always present
> > > > > > > regardless of whether we expose a vfio group interface to userspace.
> > > > > > > Remember, we create IOMMU groups even in the no-iommu case. Even
> with
> > > > > > > pure cdev, there are underlying IOMMU groups that maintain the DMA
> > > > > > > ownership.
> > > > > >
> > > > > > hmmm. As [1], when iommu is disabled, there will be no iommu_group for a
> > > > > > given device unless it is registered to VFIO, which a fake group is created.
> > > > > > That's why I hit the limitation [1]. When vfio_group is compiled out, then
> > > > > > even fake group goes away.
> > > > >
> > > > > In the vfio group case, [1] can be hit with no-iommu only when there
> > > > > are affected devices which are not bound to vfio.
> > > >
> > > > yes. because vfio would allocate fake group when device is registered to
> > > > it.
> > > >
> > > > > Why are we not
> > > > > allocating an IOMMU group to no-iommu devices when vfio group is
> > > > > disabled? Thanks,
> > > >
> > > > hmmm. when the vfio group code is configured out. The
> > > > vfio_device_set_group() just returns 0 after below patch is
> > > > applied and CONFIG_VFIO_GROUP=n. So when there is no
> > > > vfio group, the fake group also goes away.
> > > >
> > > > https://lore.kernel.org/kvm/20230401151833.124749-25-yi.l.liu@intel.com/
> > >
> > > Is this a fundamental issue or just a problem with the current
> > > implementation proposal? It seems like the latter. FWIW, I also don't
> > > see a taint happening in the cdev path for no-iommu use. Thanks,
> >
> > yes. the latter case. The reason I raised it here is to confirm the
> > policy on the new group/bdf capability in the DEVICE_GET_INFO. If
> > there is no iommu group, perhaps I only need to exclude the new
> > group/bdf capability from the cap chain of DEVICE_GET_INFO. is it?
>
> I think we need to revisit the question of why allocating an IOMMU
> group for a no-iommu device is exclusive to the vfio group support.

For no-iommu device, the iommu group is a fake group allocated by vfio.
is it? And the fake group allocation is part of the vfio group code.
It is the vfio_device_set_group() in group.c. If vfio group code is not
compiled in, vfio does not allocate fake groups. Detail for this compiling
can be found in link [1].

> We've already been down the path of trying to report a field that only
> exists for devices with certain properties with dev-id. It doesn't
> work well. I think we've said all along that while the cdev interface
> is device based, there are still going to be underlying IOMMU groups
> for the user to be aware of, they're just not as much a fundamental
> part of the interface. There should not be a case where a device
> doesn't have a group to report. Thanks,

As the patch in link [1] makes vfio group optional, so if compile a kernel
with CONFIG_VFIO_GROUP=n, and boot it with iommu disabled, then there is no
group to report. Perhaps this is not a typical usage but still a sane usage
for noiommu mode as I confirmed with you in this thread. So when it comes,
needs to consider what to report for the group field.

Perhaps I messed up the discussion by referring to a patch that is part of
another series. But I think it should be considered when talking about the
group to be reported.

[1] https://lore.kernel.org/kvm/20230401151833.124749-25-yi.l.liu@intel.com/

Regards,
Yi Liu