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Subject: [PATCH 03/30] x86, mm: document X86_CR4_PGE toggling behavior
From: Dave Hansen <dave.hansen@linux.intel.com>
Date: Wed, 08 Nov 2017 11:46:51 -0800
References: <20171108194646.907A1942@viggo.jf.intel.com>
In-Reply-To: <20171108194646.907A1942@viggo.jf.intel.com>
Message-Id: <20171108194651.47745F42@viggo.jf.intel.com>
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To: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, dave.hansen@linux.intel.com, moritz.lipp@iaik.tugraz.at, daniel.gruss@iaik.tugraz.at, michael.schwarz@iaik.tugraz.at, richard.fellner@student.tugraz.at, luto@kernel.org, torvalds@linux-foundation.org, keescook@google.com, hughd@google.com, x86@kernel.org


From: Dave Hansen <dave.hansen@linux.intel.com>

The comment says it all here.  The problem here is that the
X86_CR4_PGE bit affects all PCIDs in a way that is totally
obscure.

This makes it easier for someone to find if grepping for PCID-
related stuff and documents the hardware behavior that we are
depending on.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
Cc: Richard Fellner <richard.fellner@student.tugraz.at>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: x86@kernel.org
---

 b/arch/x86/include/asm/tlbflush.h |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff -puN arch/x86/include/asm/tlbflush.h~kaiser-prep-document-cr4-pge-behavior arch/x86/include/asm/tlbflush.h
--- a/arch/x86/include/asm/tlbflush.h~kaiser-prep-document-cr4-pge-behavior	2017-11-08 10:45:26.994681401 -0800
+++ b/arch/x86/include/asm/tlbflush.h	2017-11-08 10:45:26.997681401 -0800
@@ -257,10 +257,12 @@ static inline void __native_flush_tlb_gl
 	 */
 	WARN_ON_ONCE(!(cr4 & X86_CR4_PGE));
 	/*
-	 * Architecturally, any _change_ to X86_CR4_PGE will fully flush the
-	 * TLB of all entries including all entries in all PCIDs and all
-	 * global pages.  Make sure that we _change_ the bit, regardless of
+	 * Architecturally, any _change_ to X86_CR4_PGE will fully flush
+	 * all entries.  Make sure that we _change_ the bit, regardless of
 	 * whether we had X86_CR4_PGE set in the first place.
+	 *
+	 * Note that just toggling PGE *also* flushes all entries from all
+	 * PCIDs, regardless of the state of X86_CR4_PCIDE.
 	 */
 	native_write_cr4(cr4 ^ X86_CR4_PGE);
 	/* Put original CR4 value back: */
_

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