Re: [PATCH 3/3] x86: Switch to q35 as the default machine type

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 17, 2022 at 08:46:55AM +0100, Thomas Huth wrote:
> On 16/02/2022 18.57, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > On Wed, Feb 16, 2022 at 05:40:44PM +0000, Dr. David Alan Gilbert wrote:
> > > > * Thomas Huth (thuth@redhat.com) wrote:
> > > > > On 16/02/2022 12.01, Dr. David Alan Gilbert wrote:
> > > > > > * Gerd Hoffmann (kraxel@redhat.com) wrote:
> > > > > > >     Hi,
> > > > > > > > Given the semantic differences from 'i440fx', changing the default
> > > > > > > > machine type has effects that are equivalent to breaking command
> > > > > > > > line syntax compatibility, which is something we've always tried
> > > > > > > > to avoid.
> > > > > > > 
> > > > > > > And if we are fine breaking backward compatibility I'd rather *not* pick
> > > > > > > a default, effectively making -M $something mandatory, similar to arm.
> > > > > > 
> > > > > > Oh, that's probably easy to do;  what are other peoples thoughts on
> > > > > > that?
> > > > > 
> > > > > I agree with Gerd. Getting rid of a default machine on x86 is likely better
> > > > > than silently changing it to q35. But I'd maybe say that this should go
> > > > > through the deprecation process first?
> > > > 
> > > > So just adding something like the following under 'System emulator
> > > > machines':
> > > > 
> > > > x86 default machine type
> > > > ------------------------
> > > > 
> > > > x86 currently defaults to the very old ```pc``` machine type
> > > > which is based on the very old ```i440f``` 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 option is going to take a lot more work switching all the
> > > > test cases over; in my world here I'd only changed the tests that broke
> > > > on q35, now everything is going to need to specify a type).
> > > 
> > > ....which is still nothing compared to how many users will be impacted
> > > and the docs all over the internet we'll never be able to change, which
> > > give illustrations using qemu command lines without '-M pc'
> > 
> > What's your prreference - it sounds like you'd prefer to leave the
> > default as 'pc' ?
> > 
> > aarch's message is:
> > qemu-system-aarch64: No machine specified, and there is no default
> > Use -machine help to list supported machines
> > 
> > We could add a:
> > 'Use -machine pc for the old default behaviour'
> 
> Sounds like a good idea, but then I'd also do:
> 
> 'Use -M pc for the old default behaviour, or -M q35 for a more modern
> machine' or so.
> 
>  Thomas
> 


How about we start by going through a deprecation period then?  Make
attempts to create a machine without -machine produce a warning for a
couple of cycles.

-- 
MST