[arch-general] qemu-kvm vs. qemu upstream

C Anthony Risinger anthony at extof.me
Thu Feb 24 12:07:41 EST 2011

On Thu, Feb 24, 2011 at 10:53 AM, Jelle van der Waa <jelle at vdwaa.nl> wrote:
> On Thu, 2011-02-24 at 10:33 -0600, C Anthony Risinger wrote:
>> On Wed, Feb 23, 2011 at 7:38 PM, Allan McRae <allan at archlinux.org> wrote:
>> >
>> > If this is virtualbox specific, I'd try qemu-kvm.
>> hey,
>> i was just trying to get a concrete answer about this the other day,
>> so maybe you can clarify because i keep reading conflicting and/or
>> outdated information.
>> AFAICS, qemu-kvm is still _different_ from upstream kvm support in
>> qemu, correct?  i tried rebuilding qemu several times, ensuring i had
>> all the options i wanted (SPICE/kvm/etc) and i was getting absolutely
>> <expletive deleted> performance -- switch to qemu-kvm and she's
>> blazing again, yet many places seem to suggest they are one and the
>> same.
>> i see they definitely have different sources, but would you/anyone
>> care to elaborate on the relationship?
> qemu-kvm is  the QEMU + KVM provided by the kvm project and normal QEMU
> can use KVM as virtualizer. (correct me if i am wrong )
> from wikipedia:
>        By itself, KVM does not perform any emulation. Instead, a
>        user-space program uses the /dev/kvm interface to set up the
>        guest VM's address space, feeds it simulated I/O and maps its
>        video display back onto the host's. At least two programs
>        exploit this feature: a modified version of Qemu, and Qemu
>        itself since version 0.10.0.
> In short, if you have VT extensions, use qemu-kvm, if not use qemu since
> it emulates ( costs much cpu though ).
> QEMU/KVM is for me the best way to run windows, KVM is in the kernel so
> now rebuilding of modules, completely open source and it has nice
> features.

hmm, soo qemu doesn't actually use the VT extensions?  wtf is the
point then?  this is what i don't understand; if qemu supports KVM via
the `-enable-kvm` switch why does it suck so much -- it seems just as
slow to me as no KVM support at all.  I have a server that runs
several KVM/libvirt instances (windows being one of them purely for
... i dont even know) so i'm pretty familiar with it all, but i'm just
trying to get solid info why there is such a huge performance gap when
the both "use KVM".  i thought KVM itself did all the VT handling.

C Anthony

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