Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/10/2018 16:22, Peter Maydell wrote:
> On 5 October 2018 at 15:13, Thomas Huth <thuth@redhat.com> wrote:
>> When compiling with "--disable-tcg", we currently still use "tcg"
>> as default accelerator. "kvm" should be used in this case instead.
> 
> This part is non-controversial and makes good sense.

Though it probably should be extended to "whpx" and "hvf" (probably
"xen" too if !CONFIG_KVM).

>> Also, some downstream distros provide QEMU binaries which have "kvm"
>> in their names (e.g. "qemu-kvm" on RHEL or "kvm" on Ubuntu) that use
>> KVM by default - and some users might want to do something similar
>> with upstream binaries, too. Accomodate them by using "kvm:tcg" as
>> default when we detect such a binary name.
> 
> This part is much riskier and less clearly a good plan --
> do we really want our behaviour to vary based on the name
> of the executable? Distros who want that sort of qemu-kvm
> wrapper generally are providing it already (the Ubuntu one
> is a 2-line shell script).

I think it makes sense.  At least RHEL has qemu-kvm but no
qemu-system-x86_64, so it has a non-upstream patch to change the
accelerator; for other distros there are two benefits:

1) now: they could switch to a symlink

2) later: right now, "-accel kvm:tcg" works but we should instead switch
that to "-accel kvm -accel tcg", and deprecate "-M accel=kvm:tcg" (which
doesn't let you specify accelerator options).

I don't really like second guessing based on argv[0], because argv[0]
can actually be spoofed by the exec-ing program.  One idea could be to
pick the "best" KVM-enabled emulator that we can build, and also install
it as qemu-kvm with some magic to flip the default to kvm:tcg.  But that
can be done later; something like Thomas's patch is nice to have, and
good enough.

Paolo