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

From: Erik Rull
Subject: Re: [Qemu-devel] Current differences between qemu --enable-kvm and qemu-kvm?
Date: Tue, 22 May 2012 18:12:17 +0200
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20120429 Firefox/12.0 SeaMonkey/2.9.1

Jan Kiszka wrote:
On 2012-05-22 07:34, Jan Kiszka wrote:
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

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

I've added some more details on this to the QEMU wiki, see

BTW, if someone could have a look at the VGA diffs and resolve them,
that would be great. Gerd, what's the state of switching the BIOS?


Hi all,

thanks a lot!

I don't use PCI device assignment - so the missing irqchip-default-option should be the biggest difference between these two versions, right?

Best regards,


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