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 12/22] x86/fpu: Only write PKRU if it is different from current
Date: Thu, 21 Feb 2019 12:50:10 +0100 [thread overview]
Message-ID: <20190221115020.12385-13-bigeasy@linutronix.de> (raw)
In-Reply-To: <20190221115020.12385-1-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 43c029cdc3fe8..2d3adeb268e38 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -111,6 +111,12 @@ static inline void __write_pkru(u32 pkru)
{
u32 ecx = 0, edx = 0;
+ /*
+ * WRPKRU is relatively expensive compared to RDPKRU.
+ * Avoid WRPKRU when it would not change the value.
+ */
+ if (pkru == __read_pkru())
+ return;
/*
* "wrpkru" instruction. Loads contents in EAX to PKRU,
* requires that ecx = edx = 0.
--
2.20.1
next prev parent reply other threads:[~2019-02-21 11:50 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-21 11:49 [PATCH v7] x86: load FPU registers on return to userland Sebastian Andrzej Siewior
2019-02-21 11:49 ` [PATCH 01/22] x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 02/22] x86/fpu: Remove fpu__restore() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 03/22] x86/fpu: Remove preempt_disable() in fpu__clear() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 04/22] x86/fpu: Always init the `state' " Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 06/22] x86/fpu: Don't save fxregs for ia32 frames " Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 07/22] x86/fpu: Remove fpu->initialized Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 08/22] x86/fpu: Remove user_fpu_begin() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 09/22] x86/fpu: Add (__)make_fpregs_active helpers Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 10/22] x86/fpu: Make __raw_xsave_addr() use feature number instead of mask Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 11/22] x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() " Sebastian Andrzej Siewior
2019-02-21 11:50 ` Sebastian Andrzej Siewior [this message]
2019-02-25 18:08 ` [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current Dave Hansen
2019-03-08 17:24 ` Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 13/22] x86/pkeys: Don't check if PKRU is zero before writting it Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 14/22] x86/fpu: Eager switch PKRU state Sebastian Andrzej Siewior
2019-02-25 18:16 ` Dave Hansen
2019-03-08 18:08 ` Sebastian Andrzej Siewior
2019-03-08 19:01 ` Dave Hansen
2019-03-11 11:06 ` Sebastian Andrzej Siewior
2019-03-11 14:30 ` Sebastian Andrzej Siewior
[not found] ` <cd1a34e6-f122-33e3-864f-e23c1833a6c0@intel.com>
2019-03-21 17:10 ` Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 15/22] x86/entry: Add TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 16/22] x86/fpu: Always store the registers in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 18/22] x86/fpu: Update xstate's PKRU value on write_pkru() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 19/22] x86/fpu: Inline copy_user_to_fpregs_zeroing() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 20/22] x86/fpu: Let __fpu__restore_sig() restore the !32bit+fxsr frame from kernel memory Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 21/22] x86/fpu: Merge the two code paths in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 22/22] x86/fpu: Defer FPU state load until return to userspace Sebastian Andrzej Siewior
-- strict thread matches above, loose matches on Subject: below --
2019-01-09 11:47 [PATCH v6] x86: load FPU registers on return to userland Sebastian Andrzej Siewior
2019-01-09 11:47 ` [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current Sebastian Andrzej Siewior
2019-01-23 18:09 ` Dave Hansen
2019-02-07 11:27 ` Sebastian Andrzej Siewior
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