Re: [PATCH] hw/i386/pc_piix: Mark the machine types from version 1.4 to 1.7 as deprecated

[Thomas Huth <thuth@redhat.com>]

On 17/01/2022 21.12, Daniel P. Berrangé wrote:
> On Mon, Jan 17, 2022 at 08:16:39PM +0100, Thomas Huth wrote:
>> The list of machine types grows larger and larger each release ... and
>> it is unlikely that many people still use the very old ones for live
>> migration. QEMU v1.7 has been released more than 8 years ago, so most
>> people should have updated their machines to a newer version in those
>> 8 years at least once. Thus let's mark the very old 1.x machine types
>> as deprecated now.
> 
> What criteria did you use for picking v1.7 as the end point ?

I picked everything starting with a "1." this time ;-)

No, honestly, since we don't have a deprecation policy in place yet, there 
was no real good criteria around this time. For the machine types < 1.3 
there was a bug with migration, so these machine types could not be used for 
reliable migration anymore anyway. But for the newer machine types, we 
likely have to decide by other means indeed.

> I'm fine with the idea of aging out machine types, but I'd like us
> to explain the criteria we use for this, so that we can set clear
> expectations for users. I'm not a fan of adhoc decisions that have
> different impact every time we randomly decide to apply them.
> 
> A simple rule could be time based - eg we could say
> 
>    "we'll keep machine type versions for 5 years or 15 releases."
> 
> one factor is how long our downstream consumers have been keeping
> machines around for.
> 
> In RHEL-9 for example, the oldest machine is "pc-i440fx-rhel7.6.0"
> which IIUC is derived from QEMU 2.12.0. RHEL-9 is likely to rebase
> QEMU quite a few times over the coming years, so that 2.12.0 version
> sets an example baseline for how long machines might need to live for.
> That's 4 years this April, and could potentially be 6-7 years by the
> time RHEL-9 stops rebasing QEMU.

Yeah, 5 years still seemed a little bit short to me, that's one of the 
reasons why I did not add more machine types in my patch here. I think with 
7 or 8 years, we should be on the safe side.

Any other opinions? And if we agree on an amount of years, where should we 
document this? At the top of docs/about/deprecated.rst?

  Thomas