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

Liu, Yi L
2023-04-07T14:04:15Z

> 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/

Regards,
Yi Liu