Re: [gentoo-user] Recommendation for CPU type in QEMU?

[Marc Joliet]

Am Tue, 06 Aug 2013 14:14:32 +0100
schrieb Kerin Millar <kerframil@×××××××××××.uk>:

> On 03/08/2013 15:55, Marc Joliet wrote:
> > Am Wed, 31 Jul 2013 13:12:01 +0100
> > schrieb Kerin Millar <kerframil@×××××××××××.uk>:
> >
> >> On 31/07/2013 12:31, Marc Joliet wrote:
> >>
> >> [snip]
> >>
> >>>
> >>> There's also "-cpu host", which simply passes your CPU through to the guest.
> >>> That's what I use for my 32 bit WinXP VM. You can use it if you don't mind not
> >>> being able to migrate your guest, but it sounds to me like you're doing this on
> >>> a desktop machine, so I suspect guest migration doesn't matter to you.
> >>>
> >>
> >> I thought the same until very recently but it's not the case. The -cpu
> >> host feature exposes all feature bits supported by qemu. Those may
> >> include features that aren't supported in hardware by the host CPU, in
> >> which case qemu has to resort to (slow) emulation if they are used.
> >>
> >> --Kerin
> >
> > Just a follow up: the most authoritative answer I could find is this:
> >
> >    http://thread.gmane.org/gmane.comp.emulators.kvm.devel/84227/focus=90541
> >
> > Furthermore, the Linux KVM tuning page also defines "-cpu host" as I understand
> > it:
> >
> >    http://www.linux-kvm.org/page/Tuning_KVM
> >
> >  From the above I conclude that "-cpu host" should *not* activate CPU features
> > that the host CPU does not support.
> >
> > Otherwise I could only find out the following:
> >
> > - the Gentoo and Arch wikis both recommend "-cpu host" in conjunction with KVM
> >    (see, e.g., http://wiki.gentoo.org/wiki/QEMU/Options)
> > - in contrast, http://wiki.qemu.org/Features/CPUModels#-cpu_host_vs_-cpu_best
> >    seems to match your statement
> > - some guy on serverfault.com says this
> >    (http://serverfault.com/questions/404195/kvm-which-cpu-features-make-vms-run-better):
> >
> >    "Qemu doesn't work in the same way many other hypervisors do. For starters, it
> >    can provide full emulation. That means you can run x86 code on an ARM
> >    processor, for example. When in KVM mode, as you're using it, it doesn't
> >    actually do that... the processor is exposed no matter what, but what is
> >    reported to the OS will be changed by the -cpu flag."
> >
> >    If that's correct, "-cpu host" might mean different things when in KVM
> >    mode vs. when not. However I'm not going to blindly trust that statement.
> >
> > How/where did you find out that "-cpu host" also exposes non-host CPU features?
> >
>
> I checked the code and you're right. I had obtained the information from
> the qemu wiki but can now only assume that the content was discussing
> the feature before its implementation became concrete. Lesson to self:
> don't believe everything one reads in wikis (even official ones).
>
> --Kerin

Great, thanks for checking! Now I can be sure I'm not doing anything wrong here.

-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup