[Intel-gfx] [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

[Liu, Yi L]

Hi Alex,

> From: Alex Williamson <alex.williamson at redhat.com>
> Sent: Friday, April 7, 2023 8:04 PM
> 
> On Fri, 7 Apr 2023 10:09:58 +0000
> "Liu, Yi L" <yi.l.liu at intel.com> wrote:
> 
> > Hi Alex,
> >
> > > From: Alex Williamson <alex.williamson at redhat.com>
> > > Sent: Monday, April 3, 2023 11:02 PM
> > >
> > > On Mon, 3 Apr 2023 09:25:06 +0000
> > > "Liu, Yi L" <yi.l.liu at intel.com> wrote:
> > >
> > > > > From: Liu, Yi L <yi.l.liu at intel.com>
> > > > > Sent: Saturday, April 1, 2023 10:44 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 */
> > > >
> > > > 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.

I just realize that there is one case that does not have iommu group.
although not implemented yet. There was a discussion on SIOV support.
IIRC, it was agreed that no need to allocate iommu_group for SIOV case.
Kevin or Jason can keep me honest here. I failed to find out the link
of this discussion.

> > As another thread, we are going to add a new bdf/group capability to
> > DEVICE_GET_INFO. If the above kernel is booted, shall we exclude the new
> > bdf/group capability or add a flag in the capability to mark the group_id
> > is invalid?
> 
> As above, there's always an IOMMU group, it's never invalid.  Thanks,

Regards,
Yi Liu