Re: [PATCH v2] migration: Add documentation for backwards compatiblity

[Juan Quintela <quintela@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> wrote:
> On 11.05.23 11:27, Juan Quintela wrote:
>> State what are the requeriments to get migration working between qemu
>> versions.  And once there explain how one is supposed to implement a
>> new feature/default value and not break migration.
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> In general looks good to me:
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

Thanks.

>> ---
>> [v2]
>> - Add all danp fixes
>> [v1]
>> I will really appreciate reviews:
>> - I don't speak natively .rst format, so let me what I have done
>>    wrong.
>> - English is not my native language either (no points if had guessed
>>    that).
>
> Same for me. Sometimes your wording seems awkward to me, but I don't
> risk to propose my awkward replacement)

happens to me all the time O:-)

>> +When we do migration, we have to qemu process: the source and the
>
> two qemu processes

Done.  Already reported by daniel.

> (also probably we should say QEMU everywhere)

Done.

>> +target.  There are two cases, they are the same version or they are a
>> +different version.  The easy case is when they are the same version.
>> +The difficult one is when they are different versions.
>> +
>> +There are two things that are different, but they have very similar
>> +names and sometimes get confused:
>> +- qemu version
>> +- machine version
>> +
>> +Let's start with a practical example, we start with:
>> +
>> +- qemu-system-x86_64 (v5.2), from now on qemu-5.2.
>> +- qemu-system-x86_64 (v5.1), from now on qemu-5.1.
>> +
>> +Related to this are the "latest" machine types defined on each of
>> +them:
>> +
>> +- pc-q35-5.2 (newer one in qemu-5.2) from now on pc-5.2
>> +- pc-q35-5.1 (newer one qemu-5.1) from now on pc-5.1
>
> one in qemu-5.1

done.

>> +
>> +First of all, migration is only supposed to work if you use the same
>> +machine type in both source and destination. The qemu hardware
>> +configuration needs to be the same also on source and destination.
>> +Most aspects of the backend configuration can be changed at will,
>> +except for a few cases where the backend features influence frontend
>> +device feature exposure.  But that is not relevant for this section.
>> +
>> +I am going to list the number of combinations that we can have.  Let's
>> +start with the trivial ones, qemu is the same on source and
>> +destination:
>> +
>> +1 - qemu-5.2 -M pc-5.2  -> migrates to -> qemu-5.2 -M pc-5.2
>> +
>> +  This is the latest qemu with the latest machine type.
>> +  This have to work, and if it don't work it is a bug.
>
> doesn't

done.

Search for all don't and replace lots of them.

>> +
>> +2 - qemu-5.1 -M pc-5.1  -> migrates to -> qemu-5.1 -M pc-5.1
>> +
>> +  Exactly the same case than the previous one, but for 5.1.
>> +  Nothing to see here either.
>> +
>> +This are the easiest ones, we will not talk more about them in this
>> +section.
>> +
>> +Now we start with the more interesting cases.  Let start with the
>> +same qemu but not the same machine type.
>
> sounds like "different machine type on source and target" for me..
>
> Maybe, "not latest machine type" ?

Now we start with the more interesting cases.  Let start with a the
same QEMU process and a different QEMU version machine type.

Better?


Thanks, Juan.