Re: [PATCH v5 5/5] qemu: add default_cpu_deprecated_features configuration option

On Tue, Jul 01, 2025 at 03:58:02PM +0200, Boris Fiuczynski wrote: > On 7/1/25 10:46, Daniel P. Berrangé via Devel wrote: >> On Sun, Jun 29, 2025 at 11:19:30PM -0400, Collin Walling wrote: >>> From: Boris Fiuczynski <fiuczy(a)linux.ibm.com&gt; >>> >>> Allow to define the default for deprecated_features when the attribute >>> is not set in the cpu defintion of a domain XML. If these features are >>> still desired, they may be reenabled via the deprecated_features='on' >>> attribute. >>> >>> Some existing tests utilize this updated behavior, so update the CPU >>> features on the corresponding args files. >>> >>> Signed-off-by: Boris Fiuczynski <fiuczy(a)linux.ibm.com&gt; >>> Signed-off-by: Collin Walling <walling(a)linux.ibm.com&gt; >>> --- >>> src/qemu/libvirtd_qemu.aug | 3 ++ >>> src/qemu/qemu.conf.in | 14 ++++++++ >>> src/qemu/qemu_conf.c | 33 +++++++++++++++++++ >>> src/qemu/qemu_conf.h | 12 +++++++ >>> src/qemu/qemu_process.c | 26 ++++++++++++++- >>> src/qemu/test_libvirtd_qemu.aug.in | 1 + >>> ...deprecated-features-none.s390x-latest.args | 2 +- >>> ...default-video-type-s390x.s390x-latest.args | 2 +- >>> ...vfio-zpci-ccw-memballoon.s390x-latest.args | 2 +- >>> .../launch-security-s390-pv.s390x-latest.args | 2 +- >>> ...t-cpu-kvm-ccw-virtio-4.2.s390x-latest.args | 2 +- >>> .../s390-defaultconsole.s390x-latest.args | 2 +- >>> .../s390-panic.s390x-latest.args | 2 +- >>> 13 files changed, 95 insertions(+), 8 deletions(-) >>> >>> diff --git a/src/qemu/libvirtd_qemu.aug b/src/qemu/libvirtd_qemu.aug >>> index e1e479d72c..2b674d258d 100644 >>> --- a/src/qemu/libvirtd_qemu.aug >>> +++ b/src/qemu/libvirtd_qemu.aug >>> @@ -160,6 +160,8 @@ module Libvirtd_qemu = >>> let filesystem_entry = str_array_entry "shared_filesystems" >>> + let default_cpu_deprecated_features = str_entry "default_cpu_deprecated_features" >>> + >>> (* Entries that used to exist in the config which are now >>> * deleted. We keep on parsing them so we don't break >>> * ability to parse old configs after upgrade >>> @@ -192,6 +194,7 @@ module Libvirtd_qemu = >>> | capability_filters_entry >>> | storage_entry >>> | filesystem_entry >>> + | default_cpu_deprecated_features >>> | obsolete_entry >>> let comment = [ label "#comment" . del /#[ \t]*/ "# " . store /([^ \t\n][^\n]*)?/ . del /\n/ "\n" ] >>> diff --git a/src/qemu/qemu.conf.in b/src/qemu/qemu.conf.in >>> index 221bfa8095..368d929f78 100644 >>> --- a/src/qemu/qemu.conf.in >>> +++ b/src/qemu/qemu.conf.in >>> @@ -1100,3 +1100,17 @@ >>> # "/path/to/nvram", >>> # "/path/to/swtpm" >>> #] >>> + >>> +# If QEMU provides a list of deprecated CPU features it is possible to use >>> +# this list for removal of deprecated CPU features during CPU model expansion. >>> +# The deprecated_features XML attribute on the XML CPU element in the domain >>> +# XML can be used to turn deprecated CPU features 'off' or 'on'. Using the >>> +# option default_cpu_deprecated_features allows to define the default behavior >>> +# when the attribute deprecated_features is not provided in the domain XML. >>> +# >>> +# Possible options are: >>> +# "off" - (default) deprecated features are removed during CPU model expansion >>> +# "on" - deprecated features remain required in the expanded CPU model >>> +# "none" - no deprecated_features attribute is added to expanded CPU model >>> +# >>> +#default_cpu_deprecated_features = "off" >> >> Having a host level config parameter change the guest ABI seems like a >> bad idea to me. IMHO mgmt apps should be updated to use the XML to request >> deprecated features are turned off by default. > > it has been some time but the idea originally is yours. > https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/message/M... Sigh, I'm disagreeing with myself from a year ago :-( In that comment I was talking about host CPU model, where it is slightly acceptable for the expansion to vary over time. What I failed to contemplate when I wrote that, was that this applies to all CPU modes, even the named CPU models. > The config parameter gives customers that still make use of the deprecated > features in most of there guests the option to bail out of the default > switch without having to edit all there guests. Of course they will give up > a migrate to a CPU generation which no longer supports the cpu features to > work out of the box. > The default is switched in this patch to use deprecated_features = "off" > which disables all deprecated cpu features without using of the config > parameter. That change in defaults would effectively make the next libvirt version a regression compared to all previous versions, as CPU features would be disappearing, despite the guest config not changed meanwhile.