Re: [Qemu-devel] qemu-system-i386 vs qemu-system-x86_64 ?

[Jan Kiszka]

On 2012-09-14 11:39, Michael Tokarev wrote:
> On 14.09.2012 13:33, Daniel P. Berrange wrote:
>> On Fri, Sep 14, 2012 at 11:39:38AM +0400, Michael Tokarev wrote:
>>> What's the difference between the two except that the
>>> latter adds some more instructions (actually whole new
>>> subsytem) to the former?  Why do we need -i386, what
>>> -x86_64 does not do which does -i386?
>>
>> AFAIK,  qemu-system-x86_64 does not support KVM on a 32-bit host.
> 
> Isn't it the other way around?
> 
> Or, does qemu-system-i386 EVER support kvm?

It does support KVM on 32-bit (minus undiscovered bugs in 32-bit-only
kernel code paths once in a while - none known right now, though). Don't
confuse old qemu-kvm with QEMU here. The latter always supported KVM
with both variants, and qemu-kvm now also does as it stopped being
different.

The major difference in qemu-system-i386 vs. qemu-system-x86_64 is on
the TCG side: We measured noticeable performance benefits when running
32/16 bit OSes against qemu-system-i386 vs. using qemu-system-x86_64. I
don't have numbers at hand, but colleagues decided to use the 32-bit
version for that reason (when no KVM is available).

Jan

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