linux-kernel - Re: [Qemu-devel] [PATCH v3 01/16] Introduce probe mode for machine type none

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Date:	Thu, 5 Mar 2015 15:56:03 +0100
From:	Michael Mueller <mimu@...ux.vnet.ibm.com>
To:	Eduardo Habkost <ehabkost@...hat.com>
Cc:	Andreas Färber <afaerber@...e.de>,
	linux-s390@...r.kernel.org,
	Peter Crosthwaite <peter.crosthwaite@...inx.com>,
	kvm@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
	Alexander Graf <agraf@...e.de>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"Jason J. Herne" <jjherne@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	"Edgar E. Iglesias" <edgar.iglesias@...il.com>,
	Alistair Francis <alistair.francis@...inx.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [Qemu-devel] [PATCH v3 01/16] Introduce probe mode for machine
 type none

On Wed, 4 Mar 2015 16:19:25 -0300
Eduardo Habkost <ehabkost@...hat.com> wrote:

> On Tue, Mar 03, 2015 at 11:55:24AM +0100, Michael Mueller wrote:
> > On Mon, 02 Mar 2015 17:57:01 +0100
> > Andreas Färber <afaerber@...e.de> wrote:
> > 
> > > Am 02.03.2015 um 17:43 schrieb Michael Mueller:
> > > > On Mon, 02 Mar 2015 14:57:21 +0100
> > > > Andreas Färber <afaerber@...e.de> wrote:
> > > > 
> > > >>>  int configure_accelerator(MachineState *ms)
> > > >>>  {
> > > >>> -    const char *p;
> > > >>> +    const char *p, *name;
> > > >>>      char buf[10];
> > > >>>      int ret;
> > > >>>      bool accel_initialised = false;
> > > >>>      bool init_failed = false;
> > > >>>      AccelClass *acc = NULL;
> > > >>> +    ObjectClass *oc;
> > > >>> +    bool probe_mode = false;
> > > >>>  
> > > >>>      p = qemu_opt_get(qemu_get_machine_opts(), "accel");
> > > >>>      if (p == NULL) {
> > > >>> -        /* Use the default "accelerator", tcg */
> > > >>> -        p = "tcg";
> > > >>> +        oc = (ObjectClass *) MACHINE_GET_CLASS(current_machine);
> > > >>> +        name = object_class_get_name(oc);
> > > >>> +        probe_mode = !strcmp(name, "none" TYPE_MACHINE_SUFFIX);
> > > >>> +        if (probe_mode) {
> > > >>> +            /* Use these accelerators in probe mode, tcg should be last */
> > > >>> +            p = probe_mode_accels;
> > > >>> +        } else {
> > > >>> +            /* Use the default "accelerator", tcg */
> > > >>> +            p = "tcg";
> > > >>> +        }
> > > >>>      }  
> > > >>
> > > >> Can't we instead use an explicit ,accel=probe or ,accel=auto?
> > > >> That would then obsolete the next patch.
> > > > 
> > > > How would you express the following with the accel=<pseudo-accel> approach?
> > > > 
> > > > -probe -machine s390-ccw,accel=kvm 
> > > > 
> > > > Using machine "none" as default with tcg as last accelerator initialized should not break
> > > > anything.
> > > > 
> > > > -M none
> > > 
> > > Let me ask differently: What does -machine none or -M none have to do
> > > with probing? It reads as if you are introducing two probe modes. Why do
> > 
> > The machine none? nothing directly, I guess. What are real world use cases for that
> > machine type?
> > 
> > > you need both? If we have -probe, isn't that independent of which
> > 
> > It is just two different means to switch on the same mode.
> > 
> > > machine we specify? Who is going to call either, with which respective goal?
> > 
> > -probe itself would be sufficient but I currently do not want to enforce the use of
> > a new parameter. Best would be not to have that mode at all if possible. 
> > 
> > The intended use case is driven by management interfaces that need to draw decisions
> > on, in this particular case runnable cpu models, with information originated by qemu.
> > 
> > Let me walk through Eduardo's suggestion first and crosscheck it with my requirements
> > before we enter in a maybe afterwards obsolete discussion.
> 
> I have been working on some changes to implement x86 CPU probing code
> that creates accel objects on the fly, that may be useful. See:
>   https://github.com/ehabkost/qemu-hacks/tree/work/user-accel-init
> 
> Especially the commit:
>   kvm: Move /dev/kvm opening/closing to open/close methods
> 

So the idea is to use kvm_open/close() in the query-cpu-definitions callback on the fly without
to disturb the KVM-side data structures for the machine probe instead of going through kvm_init()
during accelerator configuration?


> The next steps I plan are:
>  * Create AccelState object on TCG too, and somehow pass it as argument
>    to cpu_x86_init()
>  * Change all kvm_enabled() occurrences on target-i386/cpu.c to use
>    the provided accel object (including
>    x86_cpu_get_supported_feature_word() and x86_cpu_filter_features())
>  * Use the new
>    x86_cpu_get_supported_feature_word()/x86_cpu_filter_features() code
>    to implement a is_runnable(X86CPUClass*, AccelState*) check
>  * Use the new is_runnable() check to extend query-cpu-definitions for x86 too
>  * Add -cpu string and machine-type arguments to the is_runnable() check
> 

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