Re: [PATCH] deprecation: x86 default machine types

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Tue, Mar 01, 2022 at 07:54:32PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Declare the intent to require a machine type to be specified on x86
> > system emulation.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  docs/about/deprecated.rst | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> > index 85773db631..143c60d105 100644
> > --- a/docs/about/deprecated.rst
> > +++ b/docs/about/deprecated.rst
> > @@ -324,6 +324,14 @@ machine is hardly emulated at all (e.g. neither the LCD nor the USB part had
> >  been implemented), so there is not much value added by this board. Use the
> >  ``ref405ep`` machine instead.
> >  
> > +x86 default machine type
> > +''''''''''''''''''''''''
> > +
> > +x86 currently defaults to the ```pc``` machine type which is based on the very
> > +old ```i440fx``` chipset.  This default will be removed and the user will be
> > +required to specify a machine type explicitly using -M; users are encouraged to
> > +switch to the not quite as old ```q35``` machine types.
> 
> This will have no impact on anyone using libvirt as a mgmt app,
> because it will explicitly set 'pc' if the user doesn't request
> a machine type.
> 
> It will, however, break a huge number of users who don't use
> libvirt or a similar mgmt app.
> 
> 'q35' is not a drop in replacement for 'pc', and even though
> it is slightly newer, the features it brings are not likely
> to be important enough for most users who aren't using a mgmt
> app to care about switching.

I can see it having advantages for those who do things like PCIe pass
through of graphics cards.
However, my main concern is that there's a split happening where
downstream we're working primarily on q35 but a lot of people still use
i440fx; eventually that split will mean the i440fx users will have a
pretty bad experience instability/features.
So I'd like to encourage them onto a35.

Ideally I'd like to make that easy; e.g. auto creating some of the PCIe
busses.

> In the ongoing work to introduce a completely new system
> emulator binary that is exclusively runtime QMP configured,
> the machine type will almost certainly be mandatory, without
> affecting existing users. That would also apply consistently
> across all target arches.

I'm assuming that will also cause the disruption to those end users.

Dave

> Overall I'm just not seeing enough benefit to justify the
> disruption we'll cause by making this change to existing
> system emulator binaries.
> 
> Regards,
> Daniel
> -- 
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> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK