linux-kernel - [tip:x86/fpu] x86/pkeys: Don't check if PKRU is zero before writing it

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Message-ID: <tip-0556cbdc2fbcb3068e5b924a8b3d5386ae0dd27d@git.kernel.org>
Date:   Sat, 13 Apr 2019 13:55:46 -0700
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     riel@...riel.com, bigeasy@...utronix.de, peterz@...radead.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, rkrcmar@...hat.com,
        luto@...nel.org, dave.hansen@...el.com, x86@...nel.org,
        mingo@...hat.com, kvm@...r.kernel.org, tglx@...utronix.de,
        mingo@...nel.org, Jason@...c4.com, bp@...e.de, pbonzini@...hat.com
Subject: [tip:x86/fpu] x86/pkeys: Don't check if PKRU is zero before writing
 it

Commit-ID:  0556cbdc2fbcb3068e5b924a8b3d5386ae0dd27d
Gitweb:     https://git.kernel.org/tip/0556cbdc2fbcb3068e5b924a8b3d5386ae0dd27d
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Wed, 3 Apr 2019 18:41:43 +0200
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Thu, 11 Apr 2019 15:41:05 +0200

x86/pkeys: Don't check if PKRU is zero before writing it

write_pkru() checks if the current value is the same as the expected
value. So instead of just checking if the current and new value is zero
(and skip the write in such a case) we can benefit from that.

Remove the zero check of PKRU, __write_pkru() provides such a check now.

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: kvm ML <kvm@...r.kernel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
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/20190403164156.19645-15-bigeasy@linutronix.de
---
 arch/x86/mm/pkeys.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
index 05bb9a44eb1c..50f65fc1b9a3 100644
--- a/arch/x86/mm/pkeys.c
+++ b/arch/x86/mm/pkeys.c
@@ -142,13 +142,6 @@ u32 init_pkru_value = PKRU_AD_KEY( 1) | PKRU_AD_KEY( 2) | PKRU_AD_KEY( 3) |
 void copy_init_pkru_to_fpregs(void)
 {
 	u32 init_pkru_value_snapshot = READ_ONCE(init_pkru_value);
-	/*
-	 * Any write to PKRU takes it out of the XSAVE 'init
-	 * state' which increases context switch cost.  Avoid
-	 * writing 0 when PKRU was already 0.
-	 */
-	if (!init_pkru_value_snapshot && !read_pkru())
-		return;
 	/*
 	 * Override the PKRU state that came from 'init_fpstate'
 	 * with the baseline from the process.