[PATCH v1 2/3] x86/msr: Switch between WRMSRNS and WRMSR with the alternatives mechanism

["Xin Li (Intel)" <xin@zytor.com>]

From: Andrew Cooper <andrew.cooper3@citrix.com>

Per the discussion about FRED MSR writes with WRMSRNS instruction [1],
use the alternatives mechanism to choose WRMSRNS when it's available,
otherwise fallback to WRMSR.

[1] https://lore.kernel.org/lkml/15f56e6a-6edd-43d0-8e83-bb6430096514@citrix.com/

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---
 arch/x86/include/asm/msr.h | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index d642037f9ed5..3e402d717815 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -99,19 +99,6 @@ static __always_inline void __wrmsr(unsigned int msr, u32 low, u32 high)
 		     : : "c" (msr), "a"(low), "d" (high) : "memory");
 }
 
-/*
- * WRMSRNS behaves exactly like WRMSR with the only difference being
- * that it is not a serializing instruction by default.
- */
-static __always_inline void __wrmsrns(u32 msr, u32 low, u32 high)
-{
-	/* Instruction opcode for WRMSRNS; supported in binutils >= 2.40. */
-	asm volatile("1: .byte 0x0f,0x01,0xc6\n"
-		     "2:\n"
-		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
-		     : : "c" (msr), "a"(low), "d" (high));
-}
-
 #define native_rdmsr(msr, val1, val2)			\
 do {							\
 	u64 __val = __rdmsr((msr));			\
@@ -312,9 +299,22 @@ do {							\
 
 #endif	/* !CONFIG_PARAVIRT_XXL */
 
+/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
+#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
+
+/* Non-serializing WRMSR, when available.  Falls back to a serializing WRMSR. */
 static __always_inline void wrmsrns(u32 msr, u64 val)
 {
-	__wrmsrns(msr, val, val >> 32);
+	/*
+	 * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant
+	 * DS prefix to avoid a trailing NOP.
+	 */
+	asm volatile("1: "
+		     ALTERNATIVE("ds wrmsr",
+				 WRMSRNS, X86_FEATURE_WRMSRNS)
+		     "2:\n"
+		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
+		     : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
 }
 
 /*
-- 
2.45.2