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Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

From: Stefano Garzarella
Subject: Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2
Date: Wed, 27 Jan 2021 15:34:21 +0100

On Wed, Jan 27, 2021 at 02:28:13PM +0000, Dr. David Alan Gilbert wrote:
* Stefano Garzarella (sgarzare@redhat.com) wrote:
On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
> * Stefano Garzarella (sgarzare@redhat.com) wrote:
> > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> > set to true by default.
> > To allow the migration, we set this property to false in the hw_compat,
> > but in the wrong place (hw_compat_4_1).
> >
> > Since commit 9d7bd0826f was released with QEMU 5.0, we move
> > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> > will have the pre-patch behavior and the migration can work.
> Be a little careful that fixing this probably causes a migration from
> 5.2->6.0 to fail with this machine type;  so when we do these
> type of fixes we often fix an old machine type between some pair of qemu
> versions and then break it between a different set.

Good point!

I did some tests using the example below always using pc-q35-4.2 and it
seems that works well:

- 5.2 -> 6.0    pass
- 5.2 -> 4.2            FAIL
- 6.0 -> 5.2    pass
- 6.0 -> 4.2    pass
- 4.2 -> 5.2    pass
- 4.2 -> 6.0    pass

Should I run some more tests?

Apologies for the delay; I had to step back and understand a bit about
what was going on.

no problem :-)

The problem here is that you're sending a 'disabled' subsection when
that option is true; your patch doesn't change the 4.1 machine type but
it does change the 4.2 machine type; and that makes the 4.2 machine type
not send it; so that means your patched version *will* work to existing
code (because it's a subsection anyway it doesn't break the stream
format when it's missing).

Thanks for the detailed explanation!

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>


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