Re: [PATCH v3 06/17] hw/sd/sdcard: Do not store vendor data on block drive (CMD56)

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jul 10, 2024 at 04:48:23PM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> 
>> > On Wed, Jul 10, 2024 at 01:21:51PM -0300, Fabiano Rosas wrote:
>> >> It's not about trust, we simply don't support migrations other than
>> >> n->n+1 and (maybe) n->n-1. So QEMU from 2016 is certainly not included.
>> >
>> > Where does it come from?  I thought we suppport that..
>> 
>> I'm taking that from:
>> 
>> docs/devel/migration/main.rst:
>>   "In general QEMU tries to maintain forward migration compatibility
>>   (i.e. migrating from QEMU n->n+1) and there are users who benefit from
>>   backward compatibility as well."
>> 
>> But of course it doesn't say whether that comes with a transitive rule
>> allowing n->n+2 migrations.
>
> I'd say that "i.e." implies n->n+1 is not the only forward migration we
> would support.
>
> I _think_ we should support all forward migration as long as the machine
> type matches.
>
>> 
>> >
>> > The same question would be: are we requesting an OpenStack cluster to
>> > always upgrade QEMU with +1 versions, otherwise migration will fail?
>> 
>> Will an OpenStack cluster be using upstream QEMU? If not, then that's a
>
> It's an example to show what I meant! :) Nothing else. Definitely not
> saying that everyone should use an upstream released QEMU (but in reality,
> it's not a problem, I think, and I do feel like people use them, perhaps
> more with the stable releases).
>
>> question for the distro. In a very practical sense, we're not requesting
>> anything. We barely test n->n+1/n->n-1, even if we had a strong support
>> statement I wouldn't be confident saying migration from QEMU 2.7 -> QEMU
>> 9.1 should succeed.
>
> No matter what we test in CI, I don't think we should break that for >1
> versions..  I hope 2.7->9.1 keeps working, otherwise I think it's legal to
> file a bug by anyone.
>
> For example, I randomly fetched a bug report:
>
> https://gitlab.com/qemu-project/qemu/-/issues/1937
>
> QEMU version:                6.2 and 7.2.5
>
> And I believe that's the common case even for upstream.  If we don't do
> that right for upstream, it can be impossible tasks for downstream and for
> all of us to maintain.

But do we do that right currently? I have no idea. Have we ever done
it? And we're here discussing a hypothetical 2.7->9.1 ...

So we cannot reuse the UNUSED field because QEMU from 2016 might send
their data and QEMU from 2024 would interpret it wrong.

How do we proceed? Add a subsection. And make the code survive when
receiving 0.

@Peter is that it? What about backwards-compat? We'll need a property as
well it seems.