linux-kernel - Re: [PATCH v3 6/7] KVM: arm64: Participate in bitmap-based PTE aging

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Message-ID: <Zgus_A_cJ68f6glV@linux.dev>
Date: Tue, 2 Apr 2024 00:00:12 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Yu Zhao <yuzhao@...gle.com>
Cc: James Houghton <jthoughton@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	David Matlack <dmatlack@...gle.com>, Marc Zyngier <maz@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Jonathan Corbet <corbet@....net>, 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>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Shaoqin Huang <shahuang@...hat.com>, Gavin Shan <gshan@...hat.com>,
	Ricardo Koller <ricarkol@...gle.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	David Rientjes <rientjes@...gle.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/7] KVM: arm64: Participate in bitmap-based PTE aging

On Tue, Apr 02, 2024 at 12:06:56AM -0400, Yu Zhao wrote:
> On Mon, Apr 1, 2024 at 7:30 PM James Houghton <jthoughton@...gle.com> wrote:
> > Suggested-by: Yu Zhao <yuzhao@...gle.com>
> 
> Thanks but I did not suggest this.

Entirely up to you, but I would still want to credit everyone who
contributed to a feature even if the underlying implementation has
changed since the original attempt.

> What I have in v2 is RCU based. I hope Oliver or someone else can help
> make that work.

Why? If there's data to show that RCU has a material benefit over taking
the MMU lock for read then I'm all for it. Otherwise, the work required
to support page-table walkers entirely outside of the MMU lock isn't
justified.

In addition to ensuring that page table teardown is always RCU-safe,
we'd need to make sure all of the walkers that take the write lock are
prepared to handle races.

> Otherwise we can just drop this for now and revisit
> later.

I really wouldn't get hung up on the locking as the make-or-break for
whether arm64 supports this MMU notifier.

-- 
Thanks,
Oliver