[PATCH 13/27] x86/fpu: Only write PKRU if it is different from current

Sebastian Andrzej Siewior
2019-04-03T16:44:12Z

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 <bigeasy@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 27328606ff687..28ffdf0c1add4 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -121,6 +121,12 @@ static inline void __write_pkru_ins(u32 pkru)

static inline void __write_pkru(u32 pkru)
{
+ /*
+ * WRPKRU is relatively expensive compared to RDPKRU.
+ * Avoid WRPKRU when it would not change the value.
+ */
+ if (pkru == __read_pkru_ins())
+ return;
__write_pkru_ins(pkru);
}

--
2.20.1