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

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Date:	Mon, 02 Mar 2015 17:57:01 +0100
From:	Andreas Färber <afaerber@...e.de>
To:	Michael Mueller <mimu@...ux.vnet.ibm.com>
CC:	qemu-devel@...gnu.org,
	"Edgar E. Iglesias" <edgar.iglesias@...il.com>,
	Peter Crosthwaite <peter.crosthwaite@...inx.com>,
	linux-s390@...r.kernel.org, kvm@...r.kernel.org,
	Gleb Natapov <gleb@...nel.org>, Alexander Graf <agraf@...e.de>,
	Eduardo Habkost <ehabkost@...hat.com>,
	linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"Jason J. Herne" <jjherne@...ux.vnet.ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.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

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
you need both? If we have -probe, isn't that independent of which
machine we specify? Who is going to call either, with which respective goal?

I think that changing the semantics of an absent ,accel=foo parameter to
mean something else than its longtime default of tcg is a bad idea.

Have you testing qtest with it? Doesn't -qtest imply accel=qtest or is
that always passed explicitly?

Regards,
Andreas

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