1 files changed, 9 insertions, 2 deletions
diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index ac0f900ebc47..1632d40e1f54 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -32,8 +32,15 @@
 int memcmp(const void *s1, const void *s2, size_t len)
 {
 	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+
+	/*
+	 * Make sure ZF is properly set in the len==0 case because in it,
+	 * RCX==0 and the REPE; CMPSB won't get executed.
+	 */
+	asm volatile("test %3, %3\n\t"
+		     "repe cmpsb"
+		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
+		     : : "cc", "memory");
 	return diff;
 }