linux-kernel - [PATCH 19/29] x86/fpu: Only write PKRU if it is different from current

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Message-Id: <20181128222035.2996-20-bigeasy@linutronix.de>
Date:   Wed, 28 Nov 2018 23:20:25 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 19/29] x86/fpu: Only write PKRU if it is different from current

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@...utronix.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)
 {
-- 
2.20.0.rc1