[vfio-users] Question about iommu groups

[Alex Williamson]

On Tue, 18 Jun 2019 11:43:58 +0100
James Courtier-Dutton <james.dutton at gmail.com> wrote:

> Hi,
> 
> In the following list of iommu groups, I am wondering why sub-functions on
> the same PCIe card are not being given the same IOMMU group as I would
> expect.

I can't provide any specifics without further details, a full 'sudo
lspci -nnvvv' listing would be required to determine if the grouping is
correct.  It is possible for PCI functions to be in separate groups if
ACS is provided to indicate isolation between the functions or if the
vendor has verified isolation exists and quirks are included in the
kernel.

> For example, I would have expected the GPU and the HMDI Audio for that GPU
> to be in the same IOMMU group.

I have yet to see a GPU that supports ACS, so my guess would be that
this is induced by the unsupported ACS override patch.  Please provide
full dmesg.
 
> I am asking, because, with the current IOMMU groups, a vfio bus reset fails.

Bus resets across multiple IOMMU groups are possible, but all of the
affected devices need to be bound to vfio-pci and either owned by the
same user or unused by other users.  If assigning all the functions to
a single VM, bus reset should work regardless of the IOMMU grouping.
Thanks,

Alex