Re: [PATCH v3 2/3] migration/docs: How to migrate when hosts have different features

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> On Mon, May 15, 2023 at 10:32:00AM +0200, Juan Quintela wrote:
>> +$ qemu-system-x86_64 -cpu host,pcid=off,stibp=off
>> +
>> +Host B:
>> +
>> +$ qemu-system-x86_64 -cpu host,taa-no=off
>
> Since we're using cpu as example, shall we at least mention at the end that
> we don't suggest using -cpu host if migration is needed?

Added something like that.

>> +
>> +And you would be able to migrate between them.  It is responsability
>> +of the management application or of the user to make sure that the
>> +configuration is correct.  QEMU don't know how to look at this kind of
>> +features in general.
>> +
>> +Other devices have worse control about individual features.  If they
>> +want to be able to migrate between hosts that show different features,
>> +the device needs a way to configure which ones it is going to use.
>> +
>> +In this section we have considered that we are using the same QEMU
>> +binary in both sides of the migration.  If we use different QEMU
>> +versions process, then we need to have into account all other
>> +differences and the examples become even more complicated.
>
> Mostly good to me.  What I worry is how much help this will bring to
> developers - I'd assume developers working on these will be aware of this.
> But I guess it's always good to have any documentation than nothing.

I have two hopes here:

- when developer finds a problem with migration, they look there.

- having a document that I can point when one of this problems happens.
  What I do now is that I write an email, normally of worse quality and
  not finding a good example.

> Acked-by: Peter Xu <peterx@redhat.com>

Thanks, Juan.