linux-kernel - [PATCH v2 2/2] KVM: arm/arm64: Skip updating PTE entry if no change

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Date:   Mon, 13 Aug 2018 10:40:49 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     kvmarm@...ts.cs.columbia.edu
Cc:     Punit Agrawal <punit.agrawal@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        suzuki.poulose@....com, marc.zyngier@....com,
        christoffer.dall@....com, stable@...r.kernel.org
Subject: [PATCH v2 2/2] KVM: arm/arm64: Skip updating PTE entry if no change

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.

Fixes: d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
Change-Id: I28e17daf394a4821b13c2cf8726bf72bf30434f9
Signed-off-by: Punit Agrawal <punit.agrawal@....com>
Cc: Marc Zyngier <marc.zyngier@....com>
Cc: Christoffer Dall <christoffer.dall@....com>
Cc: Suzuki Poulose <suzuki.poulose@....com>
Cc: stable@...r.kernel.org
---
 virt/kvm/arm/mmu.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 2ab977edc63c..d0a9dccc3793 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1120,6 +1120,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
 	/* 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))
+			goto out;
+
 		kvm_set_pte(pte, __pte(0));
 		kvm_tlb_flush_vmid_ipa(kvm, addr);
 	} else {
@@ -1127,6 +1131,7 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
 	}
 
 	kvm_set_pte(pte, *new_pte);
+out:
 	return 0;
 }
 
-- 
2.18.0