[RFC PATCH 12/14] x86/tlb: no flush on PTE change from RW->RO when PTE is clean

[Nadav Amit]

From: Nadav Amit <namit@xxxxxxxxxx>

On x86 it is possible to skip a TLB flush when a RW entry become RO and
the PTE is clean. Add logic to detect this case and skip the flush.

Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/include/asm/tlbflush.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 4f98735ab07a..58c95e36b098 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -271,8 +271,9 @@ static inline enum pte_flush_type pte_flags_flush_type(unsigned long oldflags,
 	 * dirty/access bit if needed without a fault.
 	 */
 	const pteval_t flush_on_clear = _PAGE_DIRTY | _PAGE_PRESENT |
-					_PAGE_ACCESSED | _PAGE_RW;
+					_PAGE_ACCESSED;
 	const pteval_t flush_on_set = _PAGE_NX;
+	const pteval_t flush_on_special = _PAGE_RW;
 	const pteval_t flush_on_set_relaxed = _PAGE_RW;
 	const pteval_t flush_on_clear_relaxed = _PAGE_NX;
 	const pteval_t software_flags = _PAGE_SOFTW1 | _PAGE_SOFTW2 |
@@ -302,6 +303,17 @@ static inline enum pte_flush_type pte_flags_flush_type(unsigned long oldflags,
 	if (diff & oldflags & flush_on_clear)
 		return PTE_FLUSH_STRICT;
 
+	/*
+	 * Did any of the 'flush_on_set' flags was set between 'oldflags' and
+	 * 'newflags'?
+	 */
+	if (diff & newflags & flush_on_set)
+		return PTE_FLUSH_STRICT;
+
+	/* On RW->RO, a flush is needed if the old entry is dirty */
+	if ((diff & oldflags & _PAGE_RW) && (oldflags & _PAGE_DIRTY))
+		return PTE_FLUSH_STRICT;
+
 	/* Flush on modified flags. */
 	if (diff & flush_on_change)
 		return PTE_FLUSH_STRICT;
@@ -314,7 +326,7 @@ static inline enum pte_flush_type pte_flags_flush_type(unsigned long oldflags,
 
 	/* Ensure there are no flags that were left behind */
 	if (IS_ENABLED(CONFIG_DEBUG_VM) &&
-	    (diff & ~(flush_on_clear | flush_on_set |
+	    (diff & ~(flush_on_clear | flush_on_set | flush_on_special |
 		      software_flags | flush_on_change))) {
 		VM_WARN_ON_ONCE(1);
 		return PTE_FLUSH_STRICT;
-- 
2.25.1