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

From: Igor Mammedov <imammedo@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Damien Hedde" <damien.hedde@greensocs.com>,
	"Edgar E . Iglesias" <edgar.iglesias@xilinx.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Mark Burton" <mark.burton@greensocs.com>,
	qemu-devel <qemu-devel@nongnu.org>,
	"Mirela Grujic" <mirela.grujic@greensocs.com>
Subject: Re: [RFC PATCH 0/9] Initial support for machine creation via QMP
Date: Mon, 24 May 2021 19:20:37 +0200	[thread overview]
Message-ID: <20210524192037.765d0239@redhat.com> (raw)
In-Reply-To: <CABgObfZxq1RDP12d3aCmV8drU3qFbhcUbxe4+9K_gNhXcFT-ug@mail.gmail.com>

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
> > --
> > |: https://berrange.com      -o-
> > https://www.flickr.com/photos/dberrange :|
> > |: https://libvirt.org         -o-
> > https://fstop138.berrange.com :|
> > |: https://entangle-photo.org    -o-
> > https://www.instagram.com/dberrange :|
> >
> >  