On 15 June 2017 at 17:17, Martin Kelly <mkelly@xevo.com> wrote:

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.

Right, I did some digging. Assuming you have KVM enabled then in the general case the -cpu option is irrelevant as the instructions are handled natively. -cpu just changes what CPUID flags the userspace sees, and so this is the Right Thing To Do.

Ross