x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

xen_hypercall_hvm(), which is used when running as a Xen PVH guest at
most only once during early boot, is clobbering %rbx. Depending on
whether the caller relies on %rbx to be preserved across the call or
not, this clobbering might result in an early crash of the system.

This can be avoided by using an already saved register instead of %rbx.

Fixes: b4845bb ("x86/xen: add central hypercall functions")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

Author: jgross1

arch/x86/xen/xen-head.S

@@ -117,8 +117,8 @@ SYM_FUNC_START(xen_hypercall_hvm)
 	pop %ebx
 	pop %eax
 #else
-	lea xen_hypercall_amd(%rip), %rbx
-	cmp %rax, %rbx
+	lea xen_hypercall_amd(%rip), %rcx
+	cmp %rax, %rcx
 #ifdef CONFIG_FRAME_POINTER
 	pop %rax	/* Dummy pop. */
 #endif