QEMU supports ARI on downstream ports and assigned devices may support
ARI in their extended capabilities. The endpoint ARI capability
specifies the next function, such that the OS doesn't need to walk
each possible function, however this next function is relative to the
host, not the guest. This leads to device discovery issues when we
combine separate functions into virtual multi-function packages in a
guest. For example, SR-IOV VFs are not enumerated by simply probing
the function address space, therefore the ARI next-function field is
zero. When we combine multiple VFs together as a multi-function
device in the guest, the guest OS identifies ARI is enabled, relies on
this next-function field, and stops looking for additional function
after the first is found.
Long term we should expose the ARI capability to the guest to enable
configurations with more than 8 functions per slot, but this requires
additional QEMU PCI infrastructure to manage the next-function field
for multiple, otherwise independent devices. In the short term,
hiding this capability allows equivalent functionality to what we
currently have on non-express chipsets.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
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