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

Nadav Amit
2022-07-18T19:37:51Z

From: Nadav Amit <namit@vmware.com>

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@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
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