Re: which -cpu to use

[Piavlo <piavka@cs.bgu.ac.il>]

Alexander Graf wrote:
>> 2) I want to optimize/recompile a gentoo VM system packages for the
>> chosen cpu - if that matters.
>> Does rebuilding the VM packages to fine tuned for the emulated cpu
>> give any advantages?
>
> I don't think features like SSE3 matter that much in a normal environment.
 I just wanted to rebuild the guest with following basic CGLAGS
"-march=opteron -O2 -pipe" (no sse3 and like features) tuned for my
hardware node cpu.
In Xen then i use guest linux VM both in fully virtualized mode and
paravirtual mode I see in guest's /proc/cpuinfo  the same
naitive cpu model as in hardware  node (while of course some feature
flags like svm,sse3 are missing) - but I can build the guests
with cpu native gcc -march support anyway. While seeing emulated cpu 
model in /proc/cpuinfo
 model name      : QEMU Virtual CPU version 0.9.1
is confusing and i'm not sure if i can use -march=opteron here?
> You won't get too much more benefit from choosing a different CPU type.
>> 3) Do I have to boot a VM kernel with guest viritio drivers, to get a
>> not emulated real cpu access - like I have in Xen?
>
> virtio drivers have nothing to do with CPU.
 Yes I mistakenly used the term "viritio drivers" instead of
"paravirtual guest support". So what I wanted to ask is if I build a
guest kernel with paravitual support
will it make the native hardware cpu features available inside the guest
like in Xen kernel? Or  paravirtual support is for device drivers only
and has no impact on CPU handling like in Xen?
I thought that KVM (as Xen) is a bare metal hypervisor with regards to
giving access native access to the  CPU which have  svm or vmx support,
and not just CPU emulation.
I'm confused here - can someone shed some light on my ignorance on the
matter?

 Thanks
 Alex
> They are about devices like network and block. If you want to have PV
> drivers for those, you need to have a kernel that supports virtio
> (incl. virtio_pci) and specify that you want to use virtio on the qemu
> command line as options to -net or -drive.
>
> Alex