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

[Juan Quintela <quintela@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Mon, May 15, 2023 at 10:32:00AM +0200, Juan Quintela wrote:
>> Sometimes devices have different features depending of things outside
>> of qemu.  For instance the kernel.  Document how to handle that cases.
>> 
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> yes, e.g. vhost features are exactly like this.

Yeap, I know that.  But not in enough detail to get a good example.

[...]

>> +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.
>
> How do people know what to do?

They need to:
- know their hardware/device/driver.
- how they can do it in qemu.

I can help with the second, not with the 1st.

> How about a tool that will help you get data from hosts
> and then tell you how to configure qemu to make them
> compatible?

This is the holy gray of migration.  I would like to be able to create
machines from QMP.  That way, I can transport the configuration over the
migration channel, instead of hoping that it is the same.  Troubles so
far:

- we are very far away of being able to create machines with QMP (not
  migration related).

- we have properties that can be different on source and destination.
  For instance, the path to the file that implements a device can be
  different on both systems.

  We could add some keyword to the part of the configuration that can be
  different.  As said, we can say that destination/QMP can give us a new
  path for a block device.  But we need to be able to mark number of
  CPU's as required to be the same.

- For devices/state that can't be seen from inside qemu, I don't know
  have a good idea.  It can be related that I am not an expert on that
  type of devices.  Perhaps someone that knows more about they can give
  some insights.

Later, Juan.