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

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Message-ID: <20240719145851.GA23182@willie-the-truck>
Date: Fri, 19 Jul 2024 15:58:51 +0100
From: Will Deacon <will@...nel.org>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, 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>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, ricarkol@...gle.com
Subject: Re: [PATCH] KVM: arm64: Move data barrier to end of split walk

[+Ricardo, as he wrote the original split walker]

On Thu, Jul 18, 2024 at 10:35:19PM +0000, Colton Lewis wrote:
> 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;
>  }

This looks ok to me, but it would be great if Ricardo could have a look
as well.

Will