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 | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/arch/x86/include/asm/special_insns.h
b/arch/x86/include/asm/special_insns.h
index 43c029cdc3fe8..2d3adeb268e38 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -111,6 +111,12 @@ static inline void __write_pkru(u32 pkru)
{
u32 ecx = 0, edx = 0;
+ /*
+ * WRPKRU is relatively expensive compared to RDPKRU.
+ * Avoid WRPKRU when it would not change the value.
+ */
+ if (pkru == __read_pkru())
+ return;
/*
* "wrpkru" instruction. Loads contents in EAX to PKRU,
* requires that ecx = edx = 0.
--
2.20.1
