DescriptionKashyap Chamarthy
2021-11-23 16:15:18 UTC
What?
-----
Change Nova's default video display from 'cirrus' to 'virtio'.
Why?
----
First of all, it was declared deprecated[0] back in RHEL-8.
Second, the "cirrus" display device has many limitations and is
"considered harmful"[1] by QEMU graphics maintainers since 2014.
The recommended[1] choice by the QEMU graphics maintainers is 'virtio'
(using the libvirt term) for both UEFI and BIOS guests:
- Any Linux guest with kernel 4.4 (or a kernel with the Linux
'virtio-vgpu' driver backported) can take full advantage of the
'virtio' display device.
- In case your guest kernel does _not_ have the kernel driver (called
"virtio-gpu.ko"), no problem: the 'virtio' display device will
gracefully fallback to VGA compatibility mode.
In other words, 'virtio' is a reasonable default _whether_ or not the
guest has a kernel 'virtio-gpu' driver. If the guest kernel has the
driver, it'll be used. Otherwise, it transparently falls back to the
VGA compat mode.
Lastly, the 'virtio' display device gets the most upstream
development[3], and "support for new, cool features will most likely be
added to this device."
Version requirements
--------------------
- Linux kernel version 4.4 or above
- QEMU version 2.6
Upstream Nova and its CI handsomely satisfy both the requirements:
- Nova's minimum QEMU is 4.2.0
- Ubuntu Focal, the LTS release, ships with Linux 5.4 series, so we're
good there too. (And OSes like CentOS7 have the 'virtio-gpu' driver
backported to them.)
Upgrade impact
--------------
The 'cirrus' device still exists upstream; so any Nova guest with
'cirrus' that is migrated to a compute host that has 'virtio' as the
default will migrate just fine. Upon cold-reboot (stop + start), the
Nova instance will pick up 'virtio' as the default display device.
[0] https://bugzilla.redhat.com/show_bug.cgi?id=1651994 — Declare the
"Cirrus VGA" device emulation of QEMU as deprecated in RHEL8
[1] https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/
[2] https://www.kraxel.org/blog/2019/09/display-devices-in-qemu/
[3] https://www.kraxel.org/blog/2019/09/display-devices-in-qemu/#virtio-vga
Comment 1Kashyap Chamarthy
2021-11-23 16:18:08 UTC
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (Red Hat OpenStack Platform 17.0.1 bug fix and enhancement advisory), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2023:0271
What? ----- Change Nova's default video display from 'cirrus' to 'virtio'. Why? ---- First of all, it was declared deprecated[0] back in RHEL-8. Second, the "cirrus" display device has many limitations and is "considered harmful"[1] by QEMU graphics maintainers since 2014. The recommended[1] choice by the QEMU graphics maintainers is 'virtio' (using the libvirt term) for both UEFI and BIOS guests: - Any Linux guest with kernel 4.4 (or a kernel with the Linux 'virtio-vgpu' driver backported) can take full advantage of the 'virtio' display device. - In case your guest kernel does _not_ have the kernel driver (called "virtio-gpu.ko"), no problem: the 'virtio' display device will gracefully fallback to VGA compatibility mode. In other words, 'virtio' is a reasonable default _whether_ or not the guest has a kernel 'virtio-gpu' driver. If the guest kernel has the driver, it'll be used. Otherwise, it transparently falls back to the VGA compat mode. Lastly, the 'virtio' display device gets the most upstream development[3], and "support for new, cool features will most likely be added to this device." Version requirements -------------------- - Linux kernel version 4.4 or above - QEMU version 2.6 Upstream Nova and its CI handsomely satisfy both the requirements: - Nova's minimum QEMU is 4.2.0 - Ubuntu Focal, the LTS release, ships with Linux 5.4 series, so we're good there too. (And OSes like CentOS7 have the 'virtio-gpu' driver backported to them.) Upgrade impact -------------- The 'cirrus' device still exists upstream; so any Nova guest with 'cirrus' that is migrated to a compute host that has 'virtio' as the default will migrate just fine. Upon cold-reboot (stop + start), the Nova instance will pick up 'virtio' as the default display device. [0] https://bugzilla.redhat.com/show_bug.cgi?id=1651994 — Declare the "Cirrus VGA" device emulation of QEMU as deprecated in RHEL8 [1] https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/ [2] https://www.kraxel.org/blog/2019/09/display-devices-in-qemu/ [3] https://www.kraxel.org/blog/2019/09/display-devices-in-qemu/#virtio-vga