Re: [PATCH] hw/qdev-core: Add compatibility for (non)-transitional devs

[Eduardo Habkost]

On Wed, Oct 20, 2021 at 01:02:24PM +0800, Jason Wang wrote:
> On Wed, Oct 20, 2021 at 9:31 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Wed, Oct 20, 2021 at 12:56 AM Eduardo Habkost <ehabkost@redhat.com> 
> > wrote:
> > >
> > > On Tue, Oct 19, 2021 at 12:13:17PM -0400, Michael S. Tsirkin wrote:
> > > > On Tue, Oct 19, 2021 at 11:29:13AM -0400, Eduardo Habkost wrote:
> > > > > On Tue, Oct 19, 2021 at 06:59:09AM -0400, Michael S. Tsirkin wrote:
> > > > > > On Tue, Oct 19, 2021 at 11:46:17AM +0100, Stefan Hajnoczi wrote:
> > > > > > > On Tue, Oct 12, 2021 at 10:36:01AM +0200, Jean-Louis Dupond wrote:
> > > > > > > > Forgot to CC maintainers.
> > > > > > >
> > > > > > > Also CCing Jason Wang and Michael Tsirkin for VIRTIO.
> > > > > > >
> > > > > > > Stefan
> > > > > >
> > > > > > OMG
> > > > > > where all compat properties broken all the time?
> > > > >
> > > > > Compat properties that existed when commit f6e501a28ef9 ("virtio:
> > > > > Provide version-specific variants of virtio PCI devices") was
> > > > > merged are not broken, because virtio-*-transitional and
> > > > > virtio-*-non-transitional were brand new QOM types (so there's no
> > > > > compatibility to be kept with old QEMU versions).
> > > > >
> > > > > Compat properties referencing "virtio-*-pci" instead of
> > > > > "virtio-*-pci-base" added after commit f6e501a28ef9 are probably
> > > > > broken, yes.
> > > > >
> > > > > --
> > > > > Eduardo
> > > >
> > > > Oh. So just this one:
> > > >     { "virtio-net-pci", "vectors", "3"},
> > > >
> > > > right?
> > >
> > > I think so.  That's the only post-4.0 virtio-*-pci compat property I see 
> > > in
> > > hw/core/machine.c.
> > >
> > > pc.c doesn't have any post-4.0 virtio-*-pci compat props.  I didn't see 
> > > any
> > > virtio compat props on spapr.c and s390-virtio-ccw.c.
> > >
> > > >
> > > > about the patch: how do people feel about virtio specific
> > > > stuff in qdev core? Ok by everyone?
> > >
> > > Not OK, if we have a mechanism to avoid that, already (the
> > > "virtio-net-pci-base" type name).  I wonder what we can do to
> > > make this kind of mistake less likely, though.
> > >
> > > Jean-Louis, Jason, does the following fix work?
> >
> > Yes.
> >
> > Acked-by: Jason Wang <jasowang@redhat.com>
> >
> > Thanks
> >
> > >
> > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > ---
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index b8d95eec32d..bd9c6156c1a 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -56,7 +56,7 @@ GlobalProperty hw_compat_5_2[] = {
> > >      { "ICH9-LPC", "smm-compat", "on"},
> > >      { "PIIX4_PM", "smm-compat", "on"},
> > >      { "virtio-blk-device", "report-discard-granularity", "off" },
> > > -    { "virtio-net-pci", "vectors", "3"},
> > > +    { "virtio-net-pci-base", "vectors", "3"},
> 
> Rethink about this, any chance that we can use "virtio-net-pci" as the
> base_name? It looks to me this can cause less confusion and consistent
> with the existing compat properties.

It's probably too late now: we can't change the semantics of
"-global virtio-net-pci" without breaking compatibility.

The original reasoning for making generic_name != base_name is at
this comment in struct VirtioPCIDeviceTypeInfo:

    /*
     * Common base class for the subclasses below.
     *
     * Required only if transitional_name or non_transitional_name is set.
     *
     * We need a separate base type instead of making all types
     * inherit from generic_name for two reasons:
     * 1) generic_name implements INTERFACE_PCIE_DEVICE, but
     *    transitional_name does not.
     * 2) generic_name has the "disable-legacy" and "disable-modern"
     *    properties, transitional_name and non_transitional name don't.
     */
    const char *base_name;

(I had to look it up.  I didn't remember the original reason for that)

-- 
Eduardo