Re: [Qemu-devel] [PATCH 1/2] accel: allows to select the "best" accelerator

[Thomas Huth]

On 04.10.2016 14:41, Paolo Bonzini wrote:
> 
> 
> On 04/10/2016 13:30, Laurent Vivier wrote:
>> But sometime, user wants to use a real accelerator without knowing
>> if he really can, with, for instance accel=kvm:tcg.
>> In this case, and if the accelerator is not available we
>> have a noisy "XXX accelerator not found".
>>
>> By allowing the user to ask the "best" accelerator for the given
>> target, we can avoid this problem.
>>
>> This patch introduces a new parameter for the "accel" property, the
>> "best" keyword.
>>
>> You can ask to use the best accelerator with "-M accel=best",
>> or if you want to use your favorite accelerator and if it is not
>> available, the best one, you can use, for instance
>> "-M accel=kvm:best".
> 
> I don't think there's a single definition of a "best" accelerator.  For
> example, some "-cpu" features may be available only with TCG.  In that
> case, "kvm:tcg" has a clear meaning ("kvm" if it exists, otherwise
> "tcg") but "best" doesn't.
> 
> I agree with Daniel that unit tests should use "tcg" exclusively, at
> least as a default.

Using only tcg has also some disadvantages: For some tests, it's
interesting to know whether they also work properly with KVM (e.g.
migration tests), and only using tcg by default slows down the "make
check" quite a bit - which might become an issue now that we're adding
more and more tests.

So maybe we need something like a qtest_get_accel() function that
returns "tcg" by default, but if the user set an environment variable
like QTEST_ACCEL, it uses that value instead?

 Thomas