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Re: [Qemu-devel] Current differences between qemu --enable-kvm and qemu-

From: Jan Kiszka
Subject: Re: [Qemu-devel] Current differences between qemu --enable-kvm and qemu-kvm?
Date: Tue, 22 May 2012 07:34:57 -0300
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2012-05-22 07:04, Stefan Hajnoczi wrote:
> On Mon, May 21, 2012 at 5:54 PM, Erik Rull <address@hidden> wrote:
>> is there a summary existing that shows up the rough or actual differences
>> between qemu --enable-kvm and qemu-kvm? I tested both versions with the same
>> compile and start options, the CPU performance results are identical, only
>> the bootup time of my guest system with qemu-kvm seemed to be a bit faster
>> (not measured, it just feeled so).

Current upstream does not enable the in-kernel irqchip of KVM by
default. This should explain the difference in boot-up times. Try
"-machine accel=kvm,kernel_irqchip=on". But the default will be on, just
like in qemu-kvm, once [1] is merged.

> For production KVM instances I think it still makes sense to use
> qemu-kvm packages from your distro or qemu-kvm upstream source.
> Jan Kiszka has reduced the delta between qemu.git and qemu-kvm.git to
> the point where I think the list of differences is rather small -
> maybe PCI passthrough stuff, irqfd for vhost-net (which is now also
> being upstreamed into qemu.git), and a few other things I don't know
> of.

Right, the list of differences is dramatically shrinking. As stated in
[2], soon only PCI passthrough and legacy interface dependencies on
qemu-kvm will be the remaining reasons to use it. If we are lucky, PCI
passthrough will also make it into upstream for QEMU 1.2, we are working
on this.

> For development most patches should be against qemu.git unless they
> have a dependency on qemu-kvm.git code.

Yes, unless you are working on the upstream merge itself, there is
practically no reason anymore to develop against qemu-kvm directly.


[1] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/91171
[2] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/91026

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