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

[Stefano Garzarella]

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>

Thanks,
Stefano