xen/tasklet: CFI hardening

xen/tasklet: CFI hardening

Control Flow Integrity schemes use toolchain and optionally hardware support
to help protect against call/jump/return oriented programming attacks.

Use cf_check to annotate function pointer targets for the toolchain.

The function pointer cast in hvm_vcpu_initialise() is undefined behaviour.

While it happens to function correctly, it is not compatible with control flow
typechecking, so introduce a new hvm_assert_evtchn_irq_tasklet() to handle the
parameter type conversion in a legal way.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>