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

On 7/1/25 18:35, Daniel P. Berrangé via Devel wrote: > 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. The default is only effective on s390x and if mode="host-model" is set. So that matches what you talked about.