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Date:   Thu,  8 Jun 2023 15:05:39 -0700
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Message-ID: <20230608220558.39094-2-ctshao@google.com>
Subject: [PATCH v1 2/3] KVM: arm64: Only initiate walk if page_count() > 1 in free_removed_table()
From:   Chun-Tse Shao <ctshao@google.com>
To:     linux-kernel@vger.kernel.org, yuzhao@google.com,
        oliver.upton@linux.dev
Cc:     Chun-Tse Shao <ctshao@google.com>, Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ben Gardon <bgardon@google.com>,
        Gavin Shan <gshan@redhat.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
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Page table walk is unnecessary in free_removed_table() being called from
the stage-2 unmap path while PTEs on the table is empty. It can be
fast-pathed by only initiating a walk if page_count() > 1

Original disussion can be found in:
https://lore.kernel.org/kvmarm/ZHfWzX04GlcNngdU@linux.dev/

Suggested-by: Yu Zhao <yuzhao@google.com>
Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
 arch/arm64/kvm/hyp/pgtable.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index cc1af0286755..d8e570263388 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1319,5 +1319,6 @@ void kvm_pgtable_stage2_free_removed(struct kvm_pgtable_mm_ops *mm_ops, void *pg
 		.end	= kvm_granule_size(level),
 	};
 
-	WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
+	if (mm_ops->page_count(pgtable) > 1)
+		WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
 }
-- 
2.41.0.162.gfafddb0af9-goog