[SRU][Bionic][PATCH 0/1] QEMU/KVM display is garbled when booting from kernel EFI stub due to missing bochs-drm module

Wed Apr 15 02:33:49 UTC 2020

BugLink: https://bugs.launchpad.net/bugs/1872863

[Impact]

A recent grub2 SRU, LP #1864533, now forces the kernel to boot via the kernel
EFI stub whenever EFI is enabled. This causes problems for QEMU/KVM virtual 
machines which use the VGA=std video device, as the efifb driver yields an 
unreadable garbled screen. See the attached image.

The correct framebuffer driver to use in this situation is bochs-drm, and 
modprobing it from a HWE kernel fixes the issues.

bochs-drm is missing from Bionic since CONFIG_DRM_BOCHS was disabled in LP 
#1378648 due to bochs-drm causing problems in a PowerKVM machine. This problem
appears to be fixed now, and bochs-drm has been re-enabled for Disco and up, in 
LP #1795857 and has been proven safe.

This has also come up again in LP #1823152, as well as chatter on LP #1795857
to get this enabled on Bionic.

The customer which is experiencing this issue cannot switch to VGA=qxl as a
workaround, and must use VGA=std, hence I suggest we re-enable bochs-drm for
Bionic.

[Fix]

I noticed on Focal, if you boot, the framebuffer is initially efifb:

[ 0.603716] efifb: probing for efifb
[ 0.603733] efifb: framebuffer at 0xc0000000, using 1876k, total 1875k
[ 0.603735] efifb: mode is 800x600x32, linelength=3200, pages=1
[ 0.603736] efifb: scrolling: redraw
[ 0.603738] efifb: Truecolor: size=8:8:8:8, shift=24:16:8:0
[ 0.604462] Console: switching to colour frame buffer device 100x37
[ 0.605829] fb0: EFI VGA frame buffer device 

This soon changes to bochs-drm about a second later:

[ 0.935988] bochs-drm 0000:00:01.0: remove_conflicting_pci_framebuffers: bar 0: 0xc0000000 -> 0xc0ffffff
[ 0.937023] bochs-drm 0000:00:01.0: remove_conflicting_pci_framebuffers: bar 2: 0xc1c8c000 -> 0xc1c8cfff
[ 0.937776] checking generic (c0000000 1d5000) vs hw (c0000000 1000000)
[ 0.937776] fb0: switching to bochsdrmfb from EFI VGA
[ 0.939085] Console: switching to colour dummy device 80x25
[ 0.939117] bochs-drm 0000:00:01.0: vgaarb: deactivate vga console
[ 0.939210] [drm] Found bochs VGA, ID 0xb0c5.
[ 0.939212] [drm] Framebuffer size 16384 kB @ 0xc0000000, mmio @ 0xc1c8c000.
[ 0.941955] lpc_ich 0000:00:1f.0: I/O space for GPIO uninitialized
[ 0.942069] [TTM] Zone kernel: Available graphics memory: 2006780 KiB
[ 0.942081] [TTM] Initializing pool allocator
[ 0.942089] [TTM] Initializing DMA pool allocator
[ 0.943026] virtio_blk virtio2: [vda] 20971520 512-byte logical blocks (10.7 GB/10.0 GiB)
[ 0.944019] [drm] Found EDID data blob.
[ 0.944162] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:01.0 on minor 0
[ 0.944979] fbcon: bochs-drmdrmfb (fb0) is primary device
[ 0.947712] Console: switching to colour frame buffer device 128x48 

On bionic, the framebuffer never changes from efifb, since the bochs-drm kernel
module is not built, and it is also present on the module banlist in
/etc/modprobe.d/blacklist-framebuffer.conf

bochs-drm needs to be enabled to be built in the kernel config, and removed from
the module blacklist in kmod.

[Testcase]

Create a new QEMU/KVM virtual machine, I used virt-manager. Before you install
the OS, check the box to modify settings before install. In the "Overview" tab,
enable EFI by setting the firmware to "UEFI x86_64: /usr/share/OVMF/OVMF_CODE.secboot.fd".

Set the video device to qxl while you install Bionic. Once the install is done,
reboot, do a "apt update" and "apt upgrade", to ensure you have grub2
2.02-2ubuntu8.15 installed.

Shut the VM down, and set the video device to VGA. Or VGA=std if you use the
QEMU command line.

Start the VM up, and the screen will be garbled. See attached picture.

If you install my test packages, which are available here:

https://launchpad.net/~mruffell/+archive/ubuntu/sf272653-test 

Instructions to install (on a bionic system):

1) sudo add-apt-repository ppa:mruffell/sf272653-test
2) sudo apt-get update
3) sudo apt install linux-image-unsigned-4.15.0-96-generic linux-modules-4.15.0-96-generic \
linux-modules-extra-4.15.0-96-generic linux-headers-4.15.0-96-generic linux-headers-4.15.0-96 \
libkmod2 kmod
4) sudo reboot
5) uname -rv
4.15.0-96-generic #97+TEST272653v20200409b1-Ubuntu SMP Thu Apr 9 04:09:18 UTC 2020 

If you reboot, the screen will be perfectly readable, since the bochs-drm driver
will be in use.

[Regression Potential]

We are enabling a display driver on Bionic which was previously disabled, so
there will be some virtual machines making the jump to using bochs-drm since
it would now be available.

I don't think that this is a bad thing, and in many ways will probably end up
improving some user's experience. Although there are two scenarios where
a regression could happen:

1 - frequently gamers use GPU passthrough to their kvm based virtual
machines, and I have seen them expect, or hardcode their kernel command lines
to use the efifb driver. I actually find this strange, as if you read the
kernel documentation, the efifb driver is meant for Apple Macintosh computers
only: https://www.kernel.org/doc/html/latest/fb/efifb.html
Since most gamers have to jump through a lot of hoops to get GPU passthrough
working, and most have moved on from Bionic, I don't expect this to be a problem.

2 - if the bug with PowerKVM machines isn't actually fixed in Bionic, it will 
cause a regression on POWER. We just need to test this on POWER and ensure
that the bug in LP #1378648 was fixed. If not, then we could just disable
bochs-drm on POWER, leaving it enabled for all other archs.

Matthew Ruffell (1):
  UBUNTU: [Config] Enable CONFIG_DRM_BOCHS as module for all archs

 debian.master/config/annotations          | 3 +--
 debian.master/config/config.common.ubuntu | 2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

-- 
2.25.1