linux-kernel - [PATCH 3.18 40/56] KVM: arm/arm64: Skip updating PTE entry if no change

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Date:   Mon,  3 Sep 2018 18:49:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Suzuki Poulose <suzuki.poulose@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Punit Agrawal <punit.agrawal@....com>,
        Marc Zyngier <marc.zyngier@....com>
Subject: [PATCH 3.18 40/56] KVM: arm/arm64: Skip updating PTE entry if no change

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Punit Agrawal <punit.agrawal@....com>

commit 976d34e2dab10ece5ea8fe7090b7692913f89084 upstream.

When there is contention on faulting in a particular page table entry
at stage 2, the break-before-make requirement of the architecture can
lead to additional refaulting due to TLB invalidation.

Avoid this by skipping a page table update if the new value of the PTE
matches the previous value.

Cc: stable@...r.kernel.org
Fixes: d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
Reviewed-by: Suzuki Poulose <suzuki.poulose@....com>
Acked-by: Christoffer Dall <christoffer.dall@....com>
Signed-off-by: Punit Agrawal <punit.agrawal@....com>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/arm/kvm/mmu.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -898,6 +898,10 @@ static int stage2_set_pte(struct kvm *kv
 	/* Create 2nd stage page table mapping - Level 3 */
 	old_pte = *pte;
 	if (pte_present(old_pte)) {
+		/* Skip page table update if there is no change */
+		if (pte_val(old_pte) == pte_val(*new_pte))
+			return 0;
+
 		kvm_set_pte(pte, __pte(0));
 		kvm_tlb_flush_vmid_ipa(kvm, addr);
 	} else {