[PATCH 15/23] x86/fpu: Only write PKRU if it is different from current

[Sebastian Andrzej Siewior]

Dave Hansen says that the `wrpkru' is more expensive than `rdpkru'. It
has a higher cycle cost and it's also practically a (light) speculation
barrier.

As an optimisation read the current PKRU value and only write the new
one if it is different.

Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
---
 arch/x86/include/asm/special_insns.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/special_insns.h 
b/arch/x86/include/asm/special_insns.h
index 43c029cdc3fe8..c2ccf71b22dd6 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -107,7 +107,7 @@ static inline u32 __read_pkru(void)
        return pkru;
 }
 
-static inline void __write_pkru(u32 pkru)
+static inline void __write_pkru_insn(u32 pkru)
 {
        u32 ecx = 0, edx = 0;
 
@@ -118,6 +118,17 @@ static inline void __write_pkru(u32 pkru)
        asm volatile(".byte 0x0f,0x01,0xef\n\t"
                     : : "a" (pkru), "c"(ecx), "d"(edx));
 }
+
+static inline void __write_pkru(u32 pkru)
+{
+       /*
+        * Writting PKRU is expensive. Only write the PKRU value if it is
+        * different from the current one.
+        */
+       if (pkru == __read_pkru())
+               return;
+       __write_pkru_insn(pkru);
+}
 #else
 static inline u32 __read_pkru(void)
 {
-- 
2.19.1