[PATCH 02/30] x86, tlb: Make CR4-based TLB flushes more robust

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

The existing CR4-based TLB flush currently requires global pages
to be supported *and* enabled.  But, the hardware only needs for
them to be supported.

Make the code more robust by allowing the initial state of
X86_CR4_PGE to be on *or* off.  In addition, if called in an
unexpected state (X86_CR4_PGE=0), issue a warning.  X86_CR4_PGE=0
is certainly unexpected should not be ignored it if encountered.

This essentially gives the best of both worlds: a TLB flush no
matter what, and a warning if the TLB flush is called in an
unexpected way (X86_CR4_PGE=0).

The XOR 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: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
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 |   22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 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	2017-11-10 11:22:05.534244958 -0800
+++ b/arch/x86/include/asm/tlbflush.h	2017-11-10 11:22:05.538244958 -0800
@@ -247,12 +247,24 @@ static inline void __native_flush_tlb(vo
 
 static inline void __native_flush_tlb_global_irq_disabled(void)
 {
-	unsigned long cr4;
+	unsigned long cr4 = this_cpu_read(cpu_tlbstate.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 we expect X86_CR4_PGE to be set.  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 CR4 value back: */
 	native_write_cr4(cr4);
 }
 
_