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: <ZsPLdUC44-DHKAON@google.com>
Date: Mon, 19 Aug 2024 15:47:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: Yu Zhao <yuzhao@...gle.com>, James Houghton <jthoughton@...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 Matlack <dmatlack@...gle.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 Mon, Aug 19, 2024, Oliver Upton wrote:
> On Fri, Aug 16, 2024 at 07:03:27PM -0600, Yu Zhao wrote:
> > On Fri, Aug 16, 2024 at 6:46 PM Sean Christopherson <seanjc@...gle.com> wrote:
> 
> [...]
> 
> > > Were you expecting vCPU runtime to improve (more)?  If so, lack of movement could
> > > be due to KVM arm64 taking mmap_lock for read when handling faults:
> > >
> > > https://lore.kernel.org/all/Zr0ZbPQHVNzmvwa6@google.com
> > 
> > For the above test, I don't think it's mmap_lock
> 
> Yeah, I don't think this is related to the mmap_lock.
> 
> James is likely using hardware that has FEAT_HAFDBS, so vCPUs won't
> fault for an Access flag update.

Huh, didn't know that was a thing on ARM.  Ooh, that lends even more credence to
my assertion that marking folios accessed in handle_access_fault() can go away[*].
I assume hardware-assisted updates means this code in handle_access_fault() will
no longer be hit, as KVM simply won't ever get access faults?  If so, I'll add
that info to the changelog.

	if (kvm_pte_valid(pte))
		kvm_set_pfn_accessed(kvm_pte_to_pfn(pte));

[*] https://lore.kernel.org/all/20240726235234.228822-83-seanjc@google.com

> Even if he's on a machine w/o it, Access flag faults are handled outside the
> mmap_lock.

Oh, right, they go down handle_access_fault(), not user_mem_abort().

Reviewing late Friday afternoon, never a good idea ;-)