[PATCH RFC 1/2] qemu: use "-accel" option to specify accelerator instead of "-machine"

[Jiri Denemark]

On Mon, Jan 11, 2021 at 10:14:58 +0100, Paolo Bonzini wrote:
> On 11/01/21 09:35, Ján Tomko wrote:
> >> This unfortunately cannot be done unconditionally.  You need to probe 
> >> for the availability of -accel, using something like
> > 
> > What are we probing for?
> 
> Existence of "-accel".  "-accel" allows configuration of 
> accelerator-specific, machine-independent properties.  In the past they 
> went in -machine but the two have been separate since 5.0 (with 
> backwards-compatible glue).
> 
> There were two phases of -accel support in QEMU:
> 
> - 2.9 to 4.2: only one -accel option supported; specifying a fallback 
> couldn't be done with -accel and required the older "-machine 
> accel=tcg:kvm" instead
> 
> - 5.0 or newer: multiple -accel options supported, e.g. "-accel tcg 
> -accel kvm"
> 
> and it would be possible to distinguish them, for example using QOM 
> properties. However Libvirt only ever specifies one accelerator so it 
> makes no difference.

We use a fallback for capabilities probing (in qemuProcessQMPLaunch) so
in case the old -machine type,accel=... way is going to be deprecated,
we need to be able to detect whether multiple -accel options are allowed
and use -machine otherwise.

Jirka