[PATCH v10 2/4] x86/asm, x86/boot: expose inline memcmp()

[Mauricio Faria de Oliveira]

Move the inline memcmp function currently only available in boot/string.c
into the shared string function header <asm/shared/string.h> to be reused.

This is not done through <asm/string.h> to avoid pulling unnecessary code
in boot/string.c that causes build errors in boot/compressed/string.c
and purgatory/purgatory.ro.

No functional changes.

Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>

---

Thanks to David Laight for noticing the return value difference between
inline and regular memcmp().
---
 arch/x86/boot/string.c               | 17 ++---------------
 arch/x86/include/asm/shared/string.h | 31 +++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index e10260ed58b69fc734a4e63f15785a643f414441..be454a6864225f3a972c3e81826b77ed4e8a57fe 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -15,6 +15,7 @@
 #include <linux/errno.h>
 #include <linux/limits.h>
 #include <asm/asm.h>
+#include <asm/shared/string.h>
 #include "ctype.h"
 #include "string.h"
 
@@ -31,21 +32,7 @@
 
 int memcmp(const void *s1, const void *s2, size_t len)
 {
-	bool diff;
-
-	/*
-	 * Make sure ZF is properly set in the len==0 case because in it,
-	 * RCX==0 and the REPE; CMPSB won't get executed.
-	 *
-	 * The "cc" clobber has no meaning anymore, just source compatibility.
-	 * On x86 the flag status bits are automatically added to the clobber
-	 * set when there are no =@ccXY constraints. Keep it as documentation.
-	 */
-	asm volatile("test %3, %3\n\t"
-		     "repe cmpsb"
-		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
-		     : : /* "cc", */ "memory");
-	return diff;
+	return __inline_memcmp(s1, s2, len);
 }
 
 /*
diff --git a/arch/x86/include/asm/shared/string.h b/arch/x86/include/asm/shared/string.h
new file mode 100644
index 0000000000000000000000000000000000000000..6291653fe629babf15e29060b013d357d8eacf4c
--- /dev/null
+++ b/arch/x86/include/asm/shared/string.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHARED_STRING_H
+#define _ASM_X86_SHARED_STRING_H
+
+/*
+ * Returns:	0 (equal)
+ * 		1 (not equal)
+ *
+ * In contrast, the regular memcmp() follows glibc return value semantics.
+ */
+static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_t len)
+{
+	bool diff;
+
+	/*
+	 * Make sure ZF is properly set in the len==0 case because in it,
+	 * RCX==0 and the REPE; CMPSB won't get executed.
+	 *
+	 * The "cc" clobber has no meaning anymore, just source compatibility.
+	 * On x86 the flag status bits are automatically added to the clobber
+	 * set when there are no =@ccXY constraints. Keep it as documentation.
+	 */
+	asm volatile("test %3, %3\n\t"
+		     "repe cmpsb"
+		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
+		     : : /* "cc", */ "memory");
+
+	return diff;
+}
+
+#endif /* _ASM_X86_SHARED_STRING_H */

-- 
2.47.3