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

[Daniel P. Berrange]

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 ? make check-unit doesn't
show any difference and for 'make check-qtest' the difference was
negligible (1m47 for KVM vs 1m57 for TCG).  We shouldn't be running
extensive guest workloads in unit tests, so I'd be surprised to see
a major hit in unit test time.

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://entangle-photo.org       -o-    http://search.cpan.org/~danberr/ :|