Re: [RFC PATCH 0/9] Initial support for machine creation via QMP

[Igor Mammedov]

On Fri, 14 May 2021 20:32:22 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Il ven 14 mag 2021, 18:20 Daniel P. Berrangé <berrange@redhat.com> ha
> scritto:
> 
> > My gut feeling though is accel-set would be more logical being done
> > first, as that also influences the set of features available in other
> > areas of QEMU configuration. Was there a reason you listed it after
> > machine-set ?
> >  
> 
> That was also my initial gut feeling, but actually right now the machine
> influences the accelerator more than the other way round. For example the
> initialization of the accelerator takes a machine so that for example on
> x86 the per-architecture KVM knows whether to set up SMM. Also different
> machines could use different modes for KVM (HV vs PR for ppc), and some
> machines may not be virtualizable at all so they require TCG.
> 
> The host CPU these days is really a virtualization-only synonym for -cpu
> max, which works for TCG as well. But you're right that x86 CPU flags are
> dictated by the accelerator rather than the machine, so specifying it in
> machine-set would be clumsy. On the other hand on ARM it's a bit of both:
> for KVM it's basically always -cpu host so the accelerator is important;
> but some machines may have an M profile CPU and some may have an A.
and some do not support -cpu/-smp/or rather ignore it and go ahead with 
hardcoded one/


> I don't have the sources at hand to check in which phase CPUs are created,
> but it's definitely after ACCEL_CREATED. Adding a third command
> cpu-model-set is probably the easiest way to proceed.


a naive question,
why not ditch -cpu completely and instantiate CPUs with

device_add cpu-foo,flagX=on/off

the former is just a kluge for making CLI based -smp initial_nr_cpu/-cpu foo, 
work.
I'd move that logic to CLI compat wrapper translating that to a series of 
device_add calls
for QMP based QEMU.

Also I maybe wrong, but I thought that -cpu nowadays does nothing except of 
translating
legacy cpu model name to cpu-type and flags to a bunch of '-global', which are 
applied
automatically when CPUa are created at board_init() time or later on when 
device_add is used.

> Paolo
> 
> 
> > Regards,
> > Daniel
> > --
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> >