Re: [PATCH] docs/about: Automatically deprecate versioned machine types older than 6 years

["Daniel P. Berrangé" <berrange@redhat.com>]

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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