Re: [PATCH resend] qemuboot.conf: make cpus match built artifacts

[Martin Kelly <mkelly@xevo.com>]

On 06/15/2017 04:45 AM, Burton, Ross wrote:
>
> On 2 May 2017 at 20:20, Martin Kelly <mkelly@xevo.com
> <mailto:mkelly@xevo.com>> wrote:
>
>     -QB_CPU_KVM_x86-64 = "-cpu kvm64"
>     +QB_CPU_KVM_x86-64 = "-cpu core2duo"
>
>
> What's the actual meaning of the "kvm64" and "kvm32" CPUs?  Is there a
> performance hit?  Should we be instead changing the qemu machine to
> target a different instruction set?
>

I am no expert, but running qemu -cpu help just says:

kvm32 - "Common 32-bit KVM processor"
kvm64 - "Common 64-bit KVM processor"

My best guess is that, running on qemu without kvm, both cases will be 
slow (since it's all software emulation). But, using qemu -kvm, it 
should be fast as long as you run on a native CPU that is core2duo or 
better, as the native instructions can just be exposed and passed 
through to the host. Since core2duo is very old by now, it should be 
fast for virtually everyone building on x86.

Regardless, the core issue here is not performance but merely making 
sure that binaries run without crashing in the emulated environment. We 
need to upgrade the emulation to match the binaries or downgrade the 
binaries to match the emulation (aka building against something even 
more ancient than core2duo). Further downgrading the CPU seems wrong to 
me because core2duo is already quite old.