x86/emul: Emulate %cr8 accesses

x86/emul: Emulate %cr8 accesses

Petr reports:

  (XEN) MMIO emulation failed (1): d12v1 64bit @ 0010:fffff8057ba7dfbf -> 45 0f 20 c2 ...

during introspection.

This is MOV %cr8, which is wired up for hvm_mov_{to,from}_cr(); the VMExit
fastpaths, but not for the full emulation slowpaths.

Xen's handling of %cr8 turns out to be quite wrong.  At a minimum, we need
storage for %cr8 separate to APIC_TPR, and to alter intercepts based on
whether the vLAPIC is enabled or not.  But that's more work than there is time
for in the short term, so make a stopgap fix.

Extend hvmemul_{read,write}_cr() with %cr8 cases.  Unlike hvm_mov_to_cr(),
hardware hasn't filtered out invalid values (#GP checks are ahead of
intercepts), so introduce X86_CR8_VALID_MASK.

Reported-by: Petr Beneš <w1benny@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>