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SubjectRe: [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@redhat.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@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
>

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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