Re: [Qemu-devel] [PATCH] kvm: Set default accelerator to "kvm" if the host supports it

[Markus Armbruster]

"Daniel P. Berrange" <address@hidden> writes:

> On Mon, Oct 01, 2012 at 06:43:00PM +0200, Andreas Färber wrote:
>> Hello Jan,
>> 
>> Am 01.10.2012 16:34, schrieb Jan Kiszka:
>> > If we built a target for a host that supports KVM in principle, set the
>> > default accelerator to KVM as well. This also means the start of QEMU
>> > will fail to start if KVM support turns out to be unavailable at
>> > runtime.
>> 
>> From a distro point of view this of course means that we will build
>> against KVM and that the new KVM default will start to fail for users on
>> very old hardware. Can't we do a runtime check to select the default?
>
> NB, this is *not* only about old hardware. There are plenty of users who
> use QEMU inside VMs. One very common usage I know of is image building
> tools which are run inside Amazon VMs, using libguestfs & QEMU.
>
> IMHO, default to KVM, fallback to TCG is the most friendly default
> behaviour.

Friendly perhaps, generating an infinite series of questions "why is my
guest slow as molasses?" certainly.

And for each instance of the question, there's an unknown number of
users who give QEMU a quick try, screw up KVM unknowingly, observe the
glacial speed, and conclude it's crap.