I'm doing some reviewing of XEN source code. Does
x86_emulate() (from x86_emulate.c) execute on every guest, or
is this whenever a machine doesn't have hardware assisted
virtualization?
By default, guests do not have instructions emulated. They run on
the real hardware.
There are situations which occur, (as a side effect of
virtualisation), when it traps into Xen. e.g. updating pagetables
while shadow paging is in use, a write hitting an MMIO page emulated
by Qemu, etc.
In such circumstances, the instruction which faulted must be
emulated so Xen can work out what the guest did, and apply the
appropriate action. e.g. update the pagetable as requested or
forward an io request to qemu.
Especially for the SIMD instructions, x86_emulate() is not capable
of providing a software alternative to an instruction not supported
by the hardware.