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

[Eduardo Habkost <ehabkost@redhat.com>]

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@suse.de> wrote:
> 
> > Am 02.03.2015 um 17:43 schrieb Michael Mueller:
> > > On Mon, 02 Mar 2015 14:57:21 +0100
> > > Andreas Färber <afaerber@suse.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

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

-- 
Eduardo