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

[Thomas Huth <thuth@redhat.com>]

On 04.10.2016 15:31, Daniel P. Berrange wrote:
> On Tue, Oct 04, 2016 at 02:49:21PM +0200, Thomas Huth wrote:
>> 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.
> 
> Which tests are you seeing a slow-down for ?

Well, everything that is using accel=tcg in tests/ could be accelerated.
For example, the new ipv6/ppc64 unit test is quite slow with TCG:

sudo QTEST_QEMU_BINARY=ppc64-softmmu/qemu-system-ppc64 time tests/pxe-test

 48.46user 0.07system 0:48.48elapsed

If I replace the "accel=tcg" with "accel=kvm" in pxe-test.c, the test runs
much faster:

 11.94user 0.38system 0:12.28elapsed

  Thomas