[PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust

[Dave Hansen]

Our CR4-based TLB flush currently requries global pages to be
supported *and* enabled.  But, we really only need for them to be
supported.  Make the code more robust by alllowing X86_CR4_PGE to
clear as well as set.

This change was suggested by Kirill Shutemov.

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 |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff -puN arch/x86/include/asm/tlbflush.h~kaiser-prep-make-cr4-writes-tolerate-clear-pge arch/x86/include/asm/tlbflush.h
--- a/arch/x86/include/asm/tlbflush.h~kaiser-prep-make-cr4-writes-tolerate-clear-pge	2017-10-31 15:03:49.913092716 -0700
+++ b/arch/x86/include/asm/tlbflush.h	2017-10-31 15:03:49.917092905 -0700
@@ -250,9 +250,20 @@ static inline void __native_flush_tlb_gl
 	unsigned long cr4;
 
 	cr4 = this_cpu_read(cpu_tlbstate.cr4);
-	/* clear PGE */
-	native_write_cr4(cr4 & ~X86_CR4_PGE);
-	/* write old PGE again and flush TLBs */
+	/*
+	 * This function is only called on systems that support X86_CR4_PGE
+	 * and where always set X86_CR4_PGE.  Warn if we are called without
+	 * PGE set.
+	 */
+	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
+	 * whether we had X86_CR4_PGE set in the first place.
+	 */
+	native_write_cr4(cr4 ^ X86_CR4_PGE);
+	/* Put original CR3 value back: */
 	native_write_cr4(cr4);
 }
 
_