[PATCH 14/24] x86/pkeys: Don't check if PKRU is zero before writting it

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: linux-kernel@vger.kernel.org
Cc: x86@kernel.org, "Andy Lutomirski" <luto@kernel.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
	"Rik van Riel" <riel@surriel.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Sebastian Andrzej Siewior" <bigeasy@linutronix.de>
Subject: [PATCH 14/24] x86/pkeys: Don't check if PKRU is zero before writting it
Date: Thu, 21 Mar 2019 21:26:22 +0100	[thread overview]
Message-ID: <20190321202632.16810-15-bigeasy@linutronix.de> (raw)
In-Reply-To: <20190321202632.16810-1-bigeasy@linutronix.de>

write_pkru() checks if the current value is the same as the expected
value. So instead 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 a similar check.

Signed-off-by: Sebastian Andrzej Siewior <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 05bb9a44eb1c3..50f65fc1b9a3f 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.
-- 
2.20.1

