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

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Message-ID: <ZpqDAqgbP7Rxhxt4@linux.dev>
Date: Fri, 19 Jul 2024 08:15:14 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
	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
Subject: Re: [PATCH] KVM: arm64: Move data barrier to end of split walk

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.

nitpick: in the interest of the reader, it'd be a good idea to state
explicitly what purpose the DSB serves, which is to guarantee page table
updates have been made visible to hardware table walker.

Relative ordering of table PTEs to table contents comes from the fact
that stage2_make_pte() has release semantics.

-- 
Thanks,
Oliver