Re: [PATCH preview 0/5] rust: allow minimum version of 1.83

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

On Tue, May 06, 2025 at 05:26:12PM +0800, Zhao Liu wrote:
> > > "enable Rust" supports v1.77 and "enable all devices written in Rust"
> > > supports v1.83, correct?
> > 
> > Both support v1.83 only.  However, if Rust is missing or old, "enable
> > all devices written in Rust" will fail compilation (e.g. Kconfig would
> > fail for ARM/x86 targets due to unsatisfiable CONFIG_PL011);
> 
> In this case, a brand new Rust device (without a corresponding C
> version) would be unable to compile on the above platforms which don't
> support v1.83. I'm not sure if this is an acceptable limitation or
> policy. (Has there been a similar case in history?)
> 
> > "enable Rust" will simply pick the C version of the PL011 and HPET devices.
> 
> I support this, at least the compatibility with the old QEMU won't be
> broken! Then all C devices rewritten in Rust can be covered by this
> category.

I don't really like this because it perpetuates a state where we have
parallel implementations of devices that have to be kept in sync.

If we're re-writing C devices in Rust, we need to be able to promptly
drop the C impl once the Rust impl is feature complete. Keeping 2 impls
is a general maint burden, as well as an ongoing vmstate compatibility
danger if a change in one impl is not matched by an identical change
in the other impl.

IMHO having Rust declared supported in QEMU should be aligned with being
able to drop C impls of any ported devices.

With regards,
Daniel
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