x86: fix slow int80 path after XPTI additions

For the int80 slow path to jump to handle_exception_saved, %r14 needs to
be set up suitably for XPTI purposes. This is because of the difference
in nature between the int80 path (which is synchronous WRT guest
actions) and the exception path which is potentially asynchronous.

This is XSA-259.

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

xen/arch/x86/x86_64/entry.S

@@ -405,6 +405,12 @@ int80_slow_path:
         movl  $TRAP_gp_fault,UREGS_entry_vector(%rsp)
         /* A GPF wouldn't have incremented the instruction pointer. */
         subq  $2,UREGS_rip(%rsp)
+        /*
+         * While we've cleared xen_cr3 above already, normal exception handling
+         * code has logic to restore the original value from %r15. Therefore we
+         * need to set up %r14 here, while %r15 is required to still be zero.
+         */
+        GET_STACK_END(14)
         jmp   handle_exception_saved
 
         /* create_bounce_frame & helpers don't need to be in .text.entry */