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

[Cornelia Huck <cohuck@redhat.com>]

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.

>      }
>  
>      p = accel;

> diff --git a/qemu-options.hx b/qemu-options.hx
> index 9f6e2adfff..386e6e945d 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -66,7 +66,8 @@ Supported machine properties are:
>  @table @option
>  @item accel=@var{accels1}[:@var{accels2}[:...]]
>  This is used to enable an accelerator. Depending on the target architecture,
> -kvm, xen, hax or tcg can be available. By default, tcg is used. If there is
> +kvm, xen, hax or tcg can be available. By default, the first one available
> +out of tcg, kvm, xen, hax (in that order) is used. If there is
>  more than one accelerator specified, the next one is used if the previous one
>  fails to initialize.
>  @item kernel_irqchip=on|off
> @@ -126,7 +127,8 @@ STEXI
>  @item -accel @var{name}[,prop=@var{value}[,...]]
>  @findex -accel
>  This is used to enable an accelerator. Depending on the target architecture,
> -kvm, xen, hax or tcg can be available. By default, tcg is used. If there is
> +kvm, xen, hax or tcg can be available. By default, the first one available
> +out of tcg, kvm, xen, hax (in that order) is used. If there is
>  more than one accelerator specified, the next one is used if the previous one
>  fails to initialize.
>  @table @option

These changes would still apply, as would the question about what to do
with the iotest.