[PATCH v2 1/3] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

[Juergen Gross <jgross@xxxxxxxx>]

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: b4845bb63838 ("x86/xen: add central hypercall functions")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V2:
- use %rcx instead of %rbx, keeping the lea instruction (Jan Beulich)
---
 arch/x86/xen/xen-head.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 9252652afe59..72f28d66e0e5 100644
--- a/arch/x86/xen/xen-head.S
+++ b/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
-- 
2.43.0