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

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Date:   Sat, 13 Apr 2019 13:55:04 -0700
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     bp@...e.de, riel@...riel.com, jgross@...e.com,
        linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
        tglx@...utronix.de, dave.hansen@...el.com, hpa@...or.com,
        luto@...nel.org, Jason@...c4.com, pbonzini@...hat.com,
        mingo@...nel.org, rkrcmar@...hat.com, x86@...nel.org,
        kvm@...r.kernel.org, mingo@...hat.com
Subject: [tip:x86/fpu] x86/fpu: Only write PKRU if it is different from
 current

Commit-ID:  577ff465f5a6a5a0866d75a033844810baca20a0
Gitweb:     https://git.kernel.org/tip/577ff465f5a6a5a0866d75a033844810baca20a0
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Wed, 3 Apr 2019 18:41:42 +0200
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Thu, 11 Apr 2019 15:41:05 +0200

x86/fpu: Only write PKRU if it is different from current

According to Dave Hansen, 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>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Dave Hansen <dave.hansen@...el.com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: kvm ML <kvm@...r.kernel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Rik van Riel <riel@...riel.com>
Cc: x86-ml <x86@...nel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-14-bigeasy@linutronix.de
---
 arch/x86/include/asm/special_insns.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 34897e2b52c9..0a3c4cab39db 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -121,6 +121,13 @@ static inline void wrpkru(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 == rdpkru())
+		return;
+
 	wrpkru(pkru);
 }