Re: [Qemu-devel] [PATCH RFC] accel: default to an actually available accelerator

[Kevin Wolf <kwolf@redhat.com>]

Am 06.09.2017 um 13:29 hat Cornelia Huck geschrieben:
> On Wed,  6 Sep 2017 11:49:27 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > configure_accelerator() falls back to tcg if no accelerator has
> > been specified. Formerly, we could be sure that tcg is always
> > available; however, with --disable-tcg, this is not longer true,
> > and you are not able to start qemu without explicitly specifying
> > another accelerator on those builds.
> > 
> > Instead, choose an accelerator in the order tcg->kvm->xen->hax.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> > 
> > RFC mainly because this breaks iotest 186 in a different way on a
> > tcg-less x86_64 build: Before, it fails with "-machine accel=tcg: No
> > accelerator found"; afterwards, there seems to be a difference in
> > output due to different autogenerated devices. Not sure how to handle
> > that.
> > 
> > cc:ing some hopefully interested folks (-ENOMAINTAINER again).
> > 
> > ---
> >  accel/accel.c              | 22 ++++++++++++++++++++--
> >  arch_init.c                | 17 +++++++++++++++++
> >  include/sysemu/arch_init.h |  2 ++
> >  qemu-options.hx            |  6 ++++--
> >  4 files changed, 43 insertions(+), 4 deletions(-)
> > 
> > diff --git a/accel/accel.c b/accel/accel.c
> > index 8ae40e1e13..26a3f32627 100644
> > --- a/accel/accel.c
> > +++ b/accel/accel.c
> > @@ -68,6 +68,25 @@ static int accel_init_machine(AccelClass *acc, MachineState *ms)
> >      return ret;
> >  }
> >  
> > +static const char *default_accelerator(void)
> > +{
> > +    if (tcg_available()) {
> > +        return "tcg";
> > +    }
> > +    if (kvm_available()) {
> > +        return "kvm";
> > +    }
> > +    if (xen_available()) {
> > +        return "xen";
> > +    }
> > +    if (hax_available()) {
> > +        return "hax";
> > +    }
> > +    /* configure makes sure we have at least one accelerator */
> > +    g_assert(false);
> > +    return "";
> > +}
> > +
> >  void configure_accelerator(MachineState *ms)
> >  {
> >      const char *accel, *p;
> > @@ -79,8 +98,7 @@ void configure_accelerator(MachineState *ms)
> >  
> >      accel = qemu_opt_get(qemu_get_machine_opts(), "accel");
> >      if (accel == NULL) {
> > -        /* Use the default "accelerator", tcg */
> > -        accel = "tcg";
> > +        accel = default_accelerator();
> 
> It actually may be easier to just switch the default to
> "tcg:kvm:xen:hax". Haven't tested that, though.

This would have been my first thought and looks a bit simpler, so if
it works, I'd go for it.

But the real reason why I'm replying: Should we add changing the default
to "kvm:tcg" to the list of planned 3.0 changes? I am part of the group
that intentionally uses TCG occasionally, but I think the majority of
users wants to use KVM (or whatever the fastest option is on their
system) whenever it is available.

Kevin