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