Re: Priority of -accel (was: [PATCH] tests/qemu-iotests: Update tests to recent desugarized -accel option)

[Thomas Huth <thuth@redhat.com>]

On 07/01/2020 15.37, Paolo Bonzini wrote:
> On 07/01/20 14:55, Christophe de Dinechin wrote:
>> So what about ranking the accelerators, so that all combinaisons
>> -accel=kvm:tcg, -accel=tcg:kvm, -accel kvm -accel tcg, etc would
> 
> (I assume you mean "-machine accel=kvm:tcg" and "-machine accel=tcg:kvm"
> for the first two.  This is the "older" way which has now become sugar
> for "-accel kvm -accel tcg").
> 
>> all pickup kvm if available, and tcg as a fallback? Implementation-wise,
>> it would simply mean ranking the accelerators and updating the accelerator
>> only if it’s available and better.
> 
> This is an interesting idea.  I like this better than "-accel best",
> because "-accel best" has the problem that you can't add suboptions to
> it (the suboptions for the various accelerators are disjoint).
> 
> It would break backwards compatibility for "-machine accel=tcg:kvm",
> which so far meant "use TCG if compiled in, otherwise use KVM".  This is
> not something I would have a problem with... except that "tcg:kvm" is
> the default if no -accel option is provided!

Note that we need "-M accel=tcg:kvm" (or "-accel tcg -accel kvm" now) in
tests/boot-serial-test.c for example, since some machines can't use KVM
on certain hosts (e.g. with KVM-HV on POWER8/9).

 Thomas