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

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Date: Tue, 4 Jun 2024 16:36:56 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Houghton <jthoughton@...gle.com>, 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>, 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 Tue, Jun 04, 2024, Oliver Upton wrote:
> On Tue, Jun 04, 2024 at 03:20:20PM -0700, James Houghton wrote:
> > On Fri, May 31, 2024 at 12:18 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
> > >
> > > 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:
> > > 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);
> > 
> > Oh, indeed, thank you. Just to make sure -- does it make sense to
> > retry the cmpxchg if it fails? For example, the way I have it now for
> > x86[1], we retry the cmpxchg if the spte is still a leaf, otherwise we
> > move on to the next one having done nothing. Does something like that
> > make sense for arm64?
> 
> At least for arm64 I do not see a need for retry. The only possible
> races are:
> 
>  - A stage-2 fault handler establishing / adjusting the mapping for the
>    GFN. If the guest is directly accessing the GFN in question, what's
>    the point of wiping out AF?
> 
>    Even when returning -EAGAIN we've already primed stage2_age_data::young,
>    so we report the correct state back to the primary MMU.
> 
>  - Another kvm_age_gfn() trying to age the same GFN. I haven't even
>    looked to see if this is possible from the primary MMU POV, but in
>    theory one of the calls will win the race and clear AF.
> 
> Given Yu's concerns about making pending writers wait, we should take
> every opportunity to bail on the walk.

+1.  The x86 path that retries is, for all intents and purposes, limited to Intel
CPUs that don't support EPT A/D bits, i.e. to pre-HSW CPUs.  I wouldn't make any
decisions based on that code.