linux-kernel - [PATCH] KVM: arm64: Move data barrier to end of split walk

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Message-ID: <20240718223519.1673835-1-coltonlewis@google.com>
Date: Thu, 18 Jul 2024 22:35:19 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: kvm@...r.kernel.org
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Colton Lewis <coltonlewis@...gle.com>
Subject: [PATCH] KVM: arm64: Move data barrier to end of split walk

Moving the data barrier from stage2_split_walker to after the walk is
finished in kvm_pgtable_stage2_split results in a roughly 70%
reduction in Clear Dirty Log Time in dirty_log_perf_test (modified to
use eager page splitting) when using huge pages. This gain holds
steady through a range of vcpus used (tested 1-64) and memory
used (tested 1-64GB).

This is safe to do because nothing else is using the page tables while
they are still being mapped and this is how other page table walkers
already function. None of them have a data barrier in the walker
itself.

Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
---
 arch/arm64/kvm/hyp/pgtable.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 9e2bbee77491..9788af2ca8c0 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1547,7 +1547,6 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx,
 	 */
 	new = kvm_init_table_pte(childp, mm_ops);
 	stage2_make_pte(ctx, new);
-	dsb(ishst);
 	return 0;
 }
 
@@ -1559,8 +1558,11 @@ int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size,
 		.flags	= KVM_PGTABLE_WALK_LEAF,
 		.arg	= mc,
 	};
+	int ret;
 
-	return kvm_pgtable_walk(pgt, addr, size, &walker);
+	ret = kvm_pgtable_walk(pgt, addr, size, &walker);
+	dsb(ishst);
+	return ret;
 }
 
 int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
-- 
2.45.2.1089.g2a221341d9-goog