linux-kernel - Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear

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Date:   Fri, 17 Feb 2023 09:00:20 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
        Michael Larabel <michael@...haellarabel.com>,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-mm@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear_young()

On Fri, 17 Feb 2023 04:21:28 +0000,
Yu Zhao <yuzhao@...gle.com> wrote:
> 
> On Thu, Feb 16, 2023 at 9:12 PM Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > This patch adds kvm_arch_test_clear_young() for the vast majority of
> > VMs that are not pKVM and run on hardware that sets the accessed bit
> > in KVM page tables.

I'm really interested in how you can back this statement. 90% of the
HW I have access to is not FEAT_HWAFDB capable, either because it
predates the feature or because the feature is too buggy to be useful.

Do you have numbers?

> >
> > It relies on two techniques, RCU and cmpxchg, to safely test and clear
> > the accessed bit without taking the MMU lock. The former protects KVM
> > page tables from being freed while the latter clears the accessed bit
> > atomically against both the hardware and other software page table
> > walkers.
> >
> > Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h       |  7 +++
> >  arch/arm64/include/asm/kvm_pgtable.h    |  8 +++
> >  arch/arm64/include/asm/stage2_pgtable.h | 43 ++++++++++++++
> >  arch/arm64/kvm/arm.c                    |  1 +
> >  arch/arm64/kvm/hyp/pgtable.c            | 51 ++--------------
> >  arch/arm64/kvm/mmu.c                    | 77 ++++++++++++++++++++++++-
> >  6 files changed, 141 insertions(+), 46 deletions(-)
> 
> Adding Marc and Will.
> 
> Can you please add other interested parties that I've missed?

The MAINTAINERS file has it all:

KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)
M:      Marc Zyngier <maz@...nel.org>
M:      Oliver Upton <oliver.upton@...ux.dev>
R:      James Morse <james.morse@....com>
R:      Suzuki K Poulose <suzuki.poulose@....com>
R:      Zenghui Yu <yuzenghui@...wei.com>
L:      kvmarm@...ts.linux.dev

May I suggest that you repost your patch and Cc the interested
parties yourself? I guess most folks will want to see this in context,
and not as a random, isolated change with no rationale.

	M.

-- 
Without deviation from the norm, progress is not possible.