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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 14:56:35 -0300
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2012-05-22 13:12, Erik Rull wrote:
> 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
>>>> 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.
>>> Jan
>>> [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
>> http://wiki.qemu.org/KVM.
>> 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?
>> Jan
> 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?

And the lacking MSI support when irqchip is on. If that matters depends
on your workload.


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