Re: [PATCH] deprecation: x86 default machine types

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

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.

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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