linux-kernel - [PATCH v2 15/15] powerpc/uaccess: Use asm goto for get

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Date:   Wed, 10 Mar 2021 17:46:54 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 15/15] powerpc/uaccess: Use asm goto for get_user when
 compiler supports it

clang 11 and future GCC are supporting asm goto with outputs.

Use it to implement get_user in order to get better generated code.

Note that clang requires to set x in the default branch of
__get_user_size_goto() otherwise is compliant about x not being
initialised :puzzled:

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/include/asm/uaccess.h | 55 ++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index e3e53e88cb26..960ab5c04b11 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -136,6 +136,59 @@ do {								\
 		: "=r" (err)			\
 		: "b" (uaddr), "b" (kaddr), "i" (-EFAULT), "0" (err))
 
+#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
+
+#define __get_user_asm_goto(x, addr, label, op)			\
+	asm_volatile_goto(					\
+		"1:	"op"%U1%X1 %0, %1	# get_user\n"	\
+		EX_TABLE(1b, %l2)				\
+		: "=r" (x)					\
+		: "m"UPD_CONSTR (*addr)				\
+		:						\
+		: label)
+
+#ifdef __powerpc64__
+#define __get_user_asm2_goto(x, addr, label)			\
+	__get_user_asm_goto(x, addr, label, "ld")
+#else /* __powerpc64__ */
+#define __get_user_asm2_goto(x, addr, label)			\
+	asm_volatile_goto(					\
+		"1:	lwz%X1 %0, %1\n"			\
+		"2:	lwz%X1 %L0, %L1\n"			\
+		EX_TABLE(1b, %l2)				\
+		EX_TABLE(2b, %l2)				\
+		: "=r" (x)					\
+		: "m" (*addr)					\
+		:						\
+		: label)
+#endif /* __powerpc64__ */
+
+#define __get_user_size_goto(x, ptr, size, label)				\
+do {										\
+	BUILD_BUG_ON(size > sizeof(x));						\
+	switch (size) {								\
+	case 1: __get_user_asm_goto(x, (u8 __user *)ptr, label, "lbz"); break;	\
+	case 2: __get_user_asm_goto(x, (u16 __user *)ptr, label, "lhz"); break;	\
+	case 4: __get_user_asm_goto(x, (u32 __user *)ptr, label, "lwz"); break;	\
+	case 8: __get_user_asm2_goto(x, (u64 __user *)ptr, label);  break;	\
+	default: x = 0; BUILD_BUG();						\
+	}									\
+} while (0)
+
+#define __get_user_size_allowed(x, ptr, size, retval)			\
+do {									\
+		__label__ __gus_failed;					\
+									\
+		__get_user_size_goto(x, ptr, size, __gus_failed);	\
+		retval = 0;						\
+		break;							\
+__gus_failed:								\
+		x = 0;							\
+		retval = -EFAULT;					\
+} while (0)
+
+#else /* CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
+
 #define __get_user_asm(x, addr, err, op)		\
 	__asm__ __volatile__(				\
 		"1:	"op"%U2%X2 %1, %2	# get_user\n"	\
@@ -192,6 +245,8 @@ do {								\
 		goto label;					\
 } while (0)
 
+#endif /* CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
+
 /*
  * This is a type: either unsigned long, if the argument fits into
  * that type, or otherwise unsigned long long.
-- 
2.25.0