[PATCH v5 1/3] 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'.

Note that the inline memcmp() returns 0/1, not -1/0/1 as regular memcmp()
(reported by David Laight <david.laight.linux@xxxxxxxxx>).

Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
---
 arch/x86/boot/string.c               |  6 ++----
 arch/x86/include/asm/shared/string.h | 16 ++++++++++++++++
 arch/x86/include/asm/string.h        |  1 +
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index ac0f900ebc47efa81c92e1bb2010ea41677899c4..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"
 
@@ -31,10 +32,7 @@
 
 int memcmp(const void *s1, const void *s2, size_t len)
 {
-	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
-	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..422952152f533ad75b98f3873297b39c4f5e2477
--- /dev/null
+++ b/arch/x86/include/asm/shared/string.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHARED_STRING_H
+#define _ASM_X86_SHARED_STRING_H
+
+/* Note: this memcmp() returns 0/1, not -1/0/1 as regular memcmp(). */
+static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_t len)
+{
+	bool diff;
+
+	asm("repe cmpsb"
+	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+
+	return diff;
+}
+
+#endif /* _ASM_X86_SHARED_STRING_H */
diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index 9cb5aae7fba9ffcf0f5af8f939d30467750ccaa9..f0f4fd8227bf992e78c69209efb31f0a9a0cc3b1 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -7,6 +7,7 @@
 #else
 # include <asm/string_64.h>
 #endif
+#include <asm/shared/string.h>
 
 static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
 {

-- 
2.51.0