linux-kernel - [RFC PATCH 11/14] x86/mm: use relaxed TLB flushes when protection is removed

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Date:   Mon, 18 Jul 2022 05:02:09 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>
Subject: [RFC PATCH 11/14] x86/mm: use relaxed TLB flushes when protection is removed

From: Nadav Amit <namit@...are.com>

When checking x86 PTE flags to determine whether a TLB flush is needed,
determine whether a relaxed TLB flush is sufficient. If protection is
added (NX removed or W added), indicate that a relaxed TLB flush would
suffice.

Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: David Hildenbrand <david@...hat.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will@...nel.org>
Cc: Yu Zhao <yuzhao@...gle.com>
Cc: Nick Piggin <npiggin@...il.com>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/include/asm/tlbflush.h | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 230cd1d24fe6..4f98735ab07a 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -271,18 +271,23 @@ 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_ACCESSED | _PAGE_RW;
+	const pteval_t flush_on_set = _PAGE_NX;
+	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 |
 					_PAGE_SOFTW3 | _PAGE_SOFTW4;
-	const pteval_t flush_on_change = _PAGE_RW | _PAGE_USER | _PAGE_PWT |
+	const pteval_t flush_on_change = _PAGE_USER | _PAGE_PWT |
 			  _PAGE_PCD | _PAGE_PSE | _PAGE_GLOBAL | _PAGE_PAT |
 			  _PAGE_PAT_LARGE | _PAGE_PKEY_BIT0 | _PAGE_PKEY_BIT1 |
-			  _PAGE_PKEY_BIT2 | _PAGE_PKEY_BIT3 | _PAGE_NX;
+			  _PAGE_PKEY_BIT2 | _PAGE_PKEY_BIT3;
 	unsigned long diff = oldflags ^ newflags;
 
 	BUILD_BUG_ON(flush_on_clear & software_flags);
 	BUILD_BUG_ON(flush_on_clear & flush_on_change);
 	BUILD_BUG_ON(flush_on_change & software_flags);
+	BUILD_BUG_ON(flush_on_change & flush_on_clear_relaxed);
+	BUILD_BUG_ON(flush_on_change & flush_on_set_relaxed);
 
 	/* Ignore software flags */
 	diff &= ~software_flags;
@@ -301,9 +306,16 @@ static inline enum pte_flush_type pte_flags_flush_type(unsigned long oldflags,
 	if (diff & flush_on_change)
 		return PTE_FLUSH_STRICT;
 
+	if (diff & oldflags & flush_on_clear_relaxed)
+		return PTE_FLUSH_RELAXED;
+
+	if (diff & newflags & flush_on_set_relaxed)
+		return PTE_FLUSH_RELAXED;
+
 	/* Ensure there are no flags that were left behind */
 	if (IS_ENABLED(CONFIG_DEBUG_VM) &&
-	    (diff & ~(flush_on_clear | software_flags | flush_on_change))) {
+	    (diff & ~(flush_on_clear | flush_on_set |
+		      software_flags | flush_on_change))) {
 		VM_WARN_ON_ONCE(1);
 		return PTE_FLUSH_STRICT;
 	}
-- 
2.25.1