[PATCH 05/23] x86, mm: document X86_CR4_PGE toggling behavior

[Dave Hansen]

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@xxxxxxxxxxxxxxx>
Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
---

 b/arch/x86/include/asm/tlbflush.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 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	2017-10-31 15:03:50.479119470 -0700
+++ b/arch/x86/include/asm/tlbflush.h	2017-10-31 15:03:50.482119612 -0700
@@ -258,9 +258,11 @@ 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
+	 * 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 CR3 value back: */
_