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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 13/24] x86/fpu: Only write PKRU if it is different from current
Date: Thu, 21 Mar 2019 21:26:21 +0100	[thread overview]
Message-ID: <20190321202632.16810-14-bigeasy@linutronix.de> (raw)
In-Reply-To: <20190321202632.16810-1-bigeasy@linutronix.de>

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


  parent reply	other threads:[~2019-03-21 20:29 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-21 20:26 [PATCH v8] x86: load FPU registers on return to userland Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 01/24] x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 02/24] x86/fpu: Remove fpu__restore() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 03/24] x86/fpu: Remove preempt_disable() in fpu__clear() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 04/24] x86/fpu: Always init the `state' " Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 05/24] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 06/24] x86/fpu: Don't save fxregs for ia32 frames " Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 07/24] x86/fpu: Remove fpu->initialized Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 08/24] x86/fpu: Remove user_fpu_begin() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 09/24] x86/fpu: Add (__)make_fpregs_active helpers Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 10/24] x86/fpu: Make __raw_xsave_addr() use feature number instead of mask Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 11/24] x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() " Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 12/24] x86/pkru: Provide .*_pkru_ins() functions Sebastian Andrzej Siewior
2019-03-21 20:26 ` Sebastian Andrzej Siewior [this message]
2019-03-21 20:26 ` [PATCH 14/24] x86/pkeys: Don't check if PKRU is zero before writting it Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 15/24] x86/fpu: Eager switch PKRU state Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 16/24] x86/entry: Add TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 17/24] x86/fpu: Always store the registers in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-03-31 16:54   ` Thomas Gleixner
2019-03-21 20:26 ` [PATCH 18/24] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-03-31 18:20   ` Thomas Gleixner
2019-04-01  8:24     ` Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 19/24] x86/fpu: Update xstate's PKRU value on write_pkru() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 20/24] x86/fpu: Inline copy_user_to_fpregs_zeroing() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 21/24] x86/fpu: Let __fpu__restore_sig() restore the !32bit+fxsr frame from kernel memory Sebastian Andrzej Siewior
2019-03-22 14:55   ` Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 22/24] x86/fpu: Merge the two code paths in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 23/24] x86/fpu: Defer FPU state load until return to userspace Sebastian Andrzej Siewior
2019-03-21 20:26 ` [PATCH 24/24] x86/pkeys: add PKRU value to init_fpstate Sebastian Andrzej Siewior

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