"That affects the fine detail of exactly how we translate the guest code..."
I see that the list of x86 cpus includes Nehalem, Westmere, Penryn, and many others. I was somewhat interested in what QEMU is doing differently when emulating, for example, a Nehalem versus a Westmere. From looking at target-i386/cpu.c, it appears that it is because the two microarchitectures have slight differences in the instructions they support (e.g. Westmere having the AES extensions), even though they are both x86.
I was curious about whether QEMU was doing anything that reflected differences between the two with respect to their hardware for executing standard x86 instructions, for example. From what you've said so far, I'm suspecting that it doesn't. That is what I had thought before, but when I saw the ability to specify different microarchitectures on the command line, I became curious.