Re: [PATCH] docs/about: Automatically deprecate versioned machine types older than 6 years
On Tue, Apr 30, 2024 at 10:55:38AM +0100, Daniel P. Berrangé wrote:
On Tue, Apr 30, 2024 at 08:45:29AM +0200, Thomas Huth wrote:
> Old machine types often have bugs or work-arounds that affect our
> possibilities to move forward with the QEMU code base (see for example
> https://gitlab.com/qemu-project/qemu/-/issues/2213 for a bug that likely
> cannot be fixed without breaking live migration with old machine types,
> or https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg04516.html or
> commit ea985d235b86). So instead of going through the process of manually
> deprecating old machine types again and again, let's rather add an entry
> that can stay, which declares that machine types older than 6 years are
> considered as deprecated automatically. Six years should be sufficient to
> support the release cycles of most Linux distributions.
Reading this again, I think we're mixing two concepts here.
With this 6 year cut off, we're declaring the actual *removal* date,
not the deprecation date.
A deprecation is something that happens prior to removal normally,
to give people a warning of /future/ removal, as a suggestion
that they stop using it.
If we never set the 'deprecation_reason' on a machine type, then
unless someone reads this doc, they'll never realize they are on
a deprecated machine.
When it comes to machine types, I see deprecation as a way to tell
people they should not deploy a /new/ VM on a machine type, only
use it for back compat (incoming migration / restore from saved
image) with existing deployed VMs.
If we delete a machine on the 6 year anniversary, then users
don't want to be deploying /new/ VMs using that on the
5 year anniversary as it only gives a 1 year upgrade window.
So how long far back do we consider it reasonable for a user
to deploy a /new/ VM on an old machine type ? 1 year, 2 years,
3 years ?
How about picking the half way point ? 3 years ?
ie, set deprecation_reason for any machine that is 3 years
old, but declare that our deprecation cycle lasts for
3 years, instead of the normal 1 year, when applied to
machine types.
This would give a strong hint that users should get off the
old machine type, several years before its finally deleted.
The m68k/arm archs have a nice macro for defining versions
that exposes major/minor directly. That would let us
automatically set the deprecation flag after 3 years,
avoiding manually writing patches for each release:
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 3c93c0c0a6..e40209f60a 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -101,6 +101,11 @@ static void arm_virt_compat_set(MachineClass *mc)
arm_virt_compat_len);
}
+#define MACHINE_IS_DEPRECATED(major, minor) \
+ ((QEMU_VERSION_MAJOR - major) > 3 || \
+ ((QEMU_VERSION_MAJOR - major) == 3 && \
+ (QEMU_VERSION_MINOR - minor) > 0))
+
#define DEFINE_VIRT_MACHINE_LATEST(major, minor, latest) \
static void virt_##major##_##minor##_class_init(ObjectClass *oc, \
void *data) \
@@ -109,6 +114,9 @@ static void arm_virt_compat_set(MachineClass *mc)
arm_virt_compat_set(mc); \
virt_machine_##major##_##minor##_options(mc); \
mc->desc = "QEMU " # major "." # minor " ARM Virtual
Machine"; \
+ if (MACHINE_IS_DEPRECATED(major, minor)) { \
+ mc->deprecation_reason = "machine virt-" # major "." #
minor " is not recommended for newly deployed VMs"; \
+ } \
if (latest) { \
mc->alias = "virt"; \
} \
we could easily change other arches to enable the same thing.
Then all we need do manually is the actual deletion. We would make
it a BUILD_BUG_ON after say 20 releases to force us to remember the
actual deletion at the 6 year point, without creating an immediate
build fail in that exact 18th release cycle.
With regards,
Daniel
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