linux-kernel - Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age

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Message-ID: <CALzav=daN3y9nXNuj7pPpn2u_aAQ84t161z3odP=MGLYCLfYMQ@mail.gmail.com>
Date: Fri, 30 Aug 2024 08:33:59 -0700
From: David Matlack <dmatlack@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Houghton <jthoughton@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Sean Christopherson <seanjc@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Ankit Agrawal <ankita@...dia.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, Catalin Marinas <catalin.marinas@....com>, 
	David Rientjes <rientjes@...gle.com>, James Morse <james.morse@....com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Shaoqin Huang <shahuang@...hat.com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>, 
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Thu, Aug 29, 2024 at 5:48 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> On Thu, Aug 29, 2024 at 05:33:00PM -0700, James Houghton wrote:
> > On Mon, Aug 19, 2024 at 1:42 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
> > > Asking since you had a setup / data earlier on when you were carrying
> > > the series. Hopefully with supportive data we can get arm64 to opt-in
> > > to HAVE_KVM_MMU_NOTIFIER_YOUNG_FAST_ONLY as well.
> >
> > I'll keep trying some other approaches I can take for getting similar
> > testing that Yu had; it is somewhat difficult for me to reproduce
> > those tests (and it really shouldn't be.... sorry).
>
> No need to apologize. Getting good test hardware for arm64 is a complete
> chore. Sure would love a functional workstation with cores from this
> decade...
>
> > I think it makes most sense for me to drop the arm64 patch for now and
> > re-propose it (or something stronger) alongside enabling aging. Does
> > that sound ok?
>
> I'm a bit disappointed that we haven't gotten forward progress on the
> arm64 patches, but I also recognize this is the direction of travel as
> the x86 patches are shaping up.
>
> So yeah, I'm OK with it, but I'd love to get the arm64 side sorted out
> soon while the context is still fresh.

Converting the aging notifiers to holding mmu_lock for read seems like
a pure win and minimal churn. Can we keep that patch in v7 (which
depends on the lockless notifier refactor, i.e. is not completely
stand-alone)? We can revisit enabling MGLRU on arm64 in a subsequent
series.
>
> --
> Thanks,
> Oliver