linux-kernel - [PATCH 3.16 134/328] KVM: arm/arm64: Skip updating PTE entry if no change

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Message-ID: <lsq.1544392233.968007578@decadent.org.uk>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Marc Zyngier" <marc.zyngier@....com>,
        "Christoffer Dall" <christoffer.dall@....com>,
        "Punit Agrawal" <punit.agrawal@....com>,
        "Suzuki Poulose" <suzuki.poulose@....com>
Subject: [PATCH 3.16 134/328] KVM: arm/arm64: Skip updating PTE entry if
 no change

3.16.62-rc1 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.

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>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arm/kvm/mmu.c | 4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -758,6 +758,10 @@ 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 {