linux-kernel - Re: [PATCH v4 6/7] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age

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Message-ID: <Zloicw4IU8_-V5Ns@linux.dev>
Date: Fri, 31 May 2024 12:18:11 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Ankit Agrawal <ankita@...dia.com>, Anup Patel <anup@...infault.org>,
	Atish Patra <atishp@...shpatra.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Bibo Mao <maobibo@...ngson.cn>,
	Catalin Marinas <catalin.marinas@....com>,
	David Matlack <dmatlack@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Huacai Chen <chenhuacai@...nel.org>,
	James Morse <james.morse@....com>, Jonathan Corbet <corbet@....net>,
	Marc Zyngier <maz@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shaoqin Huang <shahuang@...hat.com>, Shuah Khan <shuah@...nel.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>,
	Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
	Zenghui Yu <yuzenghui@...wei.com>, kvm-riscv@...ts.infradead.org,
	kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-mips@...r.kernel.org, linux-mm@...ck.org,
	linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	loongarch@...ts.linux.dev
Subject: Re: [PATCH v4 6/7] KVM: arm64: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Fri, May 31, 2024 at 12:11:33PM -0700, Oliver Upton wrote:
> On Wed, May 29, 2024 at 06:05:09PM +0000, James Houghton wrote:
> 
> [...]
> 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 9e2bbee77491..eabb07c66a07 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1319,10 +1319,8 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >  	data->young = true;
> >  
> >  	/*
> > -	 * stage2_age_walker() is always called while holding the MMU lock for
> > -	 * write, so this will always succeed. Nonetheless, this deliberately
> > -	 * follows the race detection pattern of the other stage-2 walkers in
> > -	 * case the locking mechanics of the MMU notifiers is ever changed.
> > +	 * This walk may not be exclusive; the PTE is permitted to change
> > +	 * from under us.
> >  	 */
> >  	if (data->mkold && !stage2_try_set_pte(ctx, new))
> >  		return -EAGAIN;
> 
> It is probably worth mentioning that if there was a race to update the
> PTE then the GFN is most likely young, so failing to clear AF probably
> isn't even consequential.

Oh, and the WARN_ON() in kvm_pgtable_stage2_test_clear_young() is bogus
now. Maybe demote it to:

  r = kvm_pgtable_walk(...);
  WARN_ON_ONCE(r && r != -EAGAIN);

-- 
Thanks,
Oliver