Am Tue, 06 Aug 2013 14:14:32 +0100 schrieb Kerin Millar <[email protected]>:
> On 03/08/2013 15:55, Marc Joliet wrote: > > Am Wed, 31 Jul 2013 13:12:01 +0100 > > schrieb Kerin Millar <[email protected]>: > > > >> 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
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