Re: [Qemu-devel] [PATCH V3] hw/virtio: Add PCIe capability to virtio devices

[Marcel Apfelbaum <marcel@redhat.com>]

On 11/05/2015 08:22 PM, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (ehabkost@redhat.com) wrote:
>> On Mon, Nov 02, 2015 at 02:12:32PM +0200, Marcel Apfelbaum wrote:
>>> On 11/02/2015 02:05 PM, Cornelia Huck wrote:
>> [...]
>>>> What's the word on compat machines and new device types, btw.? If we
>>>> fire up a compat machine, we can still specify devices that were added
>>>> with later machine versions, but of course we can't migrate to an old
>>>> machine as the device types did not exist there. Do we want to give the
>>>> user a hint here by disallowing new device types?
>>>>
>>>
>>> I started to wonder about this too, so I added to this thread the migration
>>> maintainers that should be qualified to answer this :)
>>
>> This looks no different from all other features that are available on
>> newer QEMU versions and prevent migration to older hosts (even ones that
>> are not guest-visible, like backend configuration). Management can
>> easily detect the unavailability of those features in other hosts, long
>> before trying migration (and have better ways to warn the user if
>> necessary).
>>
>> Also, it looks like a potential nightmare for downstream maintainers
>> that cherry-pick and rebase patches, so I hope we don't consider
>> implementing that. :)
>
>     a) It's fine to add new devices and allow them to be used with old machine
>        types
>     b) The rule is that any old machine type used in the way it used to be used
>        must stay the same.
>     c) That also means it's fine to add new features that can be turned on
>        with old machine types; as long as the default is that they behave
>        just like they always did.
>     d) If you know a new device just isn't going to work with an old
>        machine type then please make it fail early with an obvious
>        error.
>
> Having said all that; I have seen requests for some magic which would
> tell the management tool whether something is 'safe for migration';
> so imagine that a user has a pile of hosts, some of which have qemu 2.n on
> and some have qemu 2.n+1 ; if he creates his VM on 2.n+1 and uses
> a feature that's new in 2.n+1 the management tool can't warn him
> because they've not yet expressed an interest in migrating to
> the 2.n machine.

Exactly, so how can I do (d) ?
A "migration possible" machine mapping qemu-pc-2.x -> qemu-pc-2.y is not enough, we need to
compare also the QEMU versions and have a "minimum QEMU version per feature."
Do we have a way to do this today in QEMU?

Thanks,
Marcel

>
> Dave
>
>>
>> --
>> Eduardo
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>