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

[Thomas Huth <thuth@redhat.com>]

On 08/01/2020 11.39, Alex Bennée wrote:
> 
> Thomas Huth <thuth@redhat.com> writes:
> 
>> On 07/01/2020 13.54, Daniel P. Berrangé wrote:
>>> On Tue, Jan 07, 2020 at 01:23:18PM +0100, Paolo Bonzini wrote:
>>>> On 07/01/20 13:18, Thomas Huth wrote:
>>>>> I don't think we need a separate priority parameter here. But IMHO it's
>>>>>  really rather common practice to prioritize the last option. So while
>>>>> it might be more "self-explanatory" to a CLI newbie if the first
>>>>> occurrence got the highest priority, it might be rather confusing
>>>>> instead for a CLI veteran...?
>>>>
>>>> Prioritising the last certainly makes sense for a choose-one-only
>>>> option, but I'm not sure it's the same for a choose-best option.  After
>>>> all it was -machine accel=kvm:tcg, not -machine accel=tcg:kvm...
>>>
>>> IIUC, the main use case for specifying multiple accelerators is
>>> so that lazy invokations can ask for a hardware virt, but then get
>>> fallback to TCG if not available. For things that should be platform
>>> portabile, there's more than just kvm to consider though, as we have
>>> many accelerators.  Listing all possible accelerators is kind of
>>> crazy though no matter what the syntax is.
>>>
>>> How about taking a completely different approach, inspired by the
>>> -cpu arg and implement:
>>>
>>>     -machine accel=best
>>
>> Something like that sounds like the best solution to me, but I'd maybe
>> rather not call it "best", since the definition of "best" might depend
>> on your use-case (e.g. do you want to use a CPU close to the host or
>> something different which might be better emulated by TCG?).
> 
> Indeed - you may well want to do TCG on Aarch64 if you want to test new
> instructions.
> 
>>
>> What about "-accel any" or "-accel fastest" or something similar?
> 
> "any" is just ambiguous, "fastest" is just begging for me to find a
> micro-benchmark that TCG outperforms on ;-)
> 
> "-accel default" could be considered to have vibes of Do The Right
> Thing (tm) and could in time actually become so!

"-accel default" sounds like the default behavior that you'd also get if
you don't use this option at all ... what about "-accel auto" to say
that QEMU should pick an accelerator automatically?

 Thomas