[vfio-users] Bisected an issue with OVMF

[Samuel Holland]

Hello,

I've been using vfio to pass through a GPU[1] to a QEMU virtual machine
for several years. I've been using OVMF since some time in 2015.
Initially, I updated my OVMF binary from Gerd's repo on a regular basis,
but some time around February 2016, the OVMF binaries stopped booting. I
could pass through other devices (EHCI controllers, audio) fine, but if
I attached the GPU, I would have one core stuck at 100% usage
indefinitely, and a blank screen and no serial console output from OVMF.

The OVMF hang would happen regardless of the number of cores, or the
amount of RAM given to the VM, or whether or not the GPU was attached to
a root port.

By the time I got around to investigating the issue, I'd forgotten the
date of the last good revision (I only had the fd images on hand, not
the RPM), and nobody else had heard of my issue. This spring, I tried
compiling various revisions of the edk2 source repo, but none of them
booted.

Thanks to the recent thread where a couple of old OVMF builds were
posted, I was finally able to bisect the issue! The build[2] from 2015
booted! The one from mid-2016 did not, as expected. Anyway, I tracked
the breakage down to this commit:

commit 7daf2401d420573f50e3d00ae3a89e54914ef056
Author: Laszlo Ersek <lersek at redhat.com>
Date:   Fri Mar 4 01:49:54 2016 +0100

     OvmfPkg: PciHostBridgeLib: permit access to the full extended 
config space

     By now OVMF makes MdeModulePkg/Bus/Pci/PciHostBridgeDxe go through
     MMCONFIG (when running on Q35). Enable the driver to address each 
B/D/F's
     config space up to and including offset 0xFFF.

With that commit reverted, current edk2 master (2c2c68b9d3e8) also boots
successfully. I don't know if there's a bug somewhere in the software
stack, or if my graphics card has a bug and needs a quirk, or what the
issue is. But I'd love to know why this doesn't work, and get whatever's
broken fixed.

For what it's worth, back when I first started using this GPU and
motherboard[3] combination, I had to manually apply this patch[4] to
QEMU for assignment to work, until it was included in a released version
of QEMU.

Thanks,
Samuel

[1]: https://www.gigabyte.com/Graphics-Card/GV-N660OC-2GD
[2]: 
https://github.com/jkoelndorfer/local-tools/blob/master/workstation/vfio/edk2.git-ovmf-x64-0-20150804.b1143.g8ca1489.noarch.rpm
[3]: http://www.asrock.com/mb/Intel/Z97E-ITXac/
[4]: 
https://github.com/qemu/qemu/commit/f5793fd9e1fd89808f4adbfe690235b094176a37