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

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

On Fri, 7 Apr 2023 10:09:58 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> Hi Alex,
> 
> > From: Alex Williamson <alex.williamson@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@intel.com> wrote:
> >   
> > > > From: Liu, Yi L <yi.l.liu@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.

> 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,

Alex