linux-kernel - Re: [PATCH v6 08/11] KVM: x86: Optimize kvm_{test_,}age

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Message-ID: <Zr_2Qhflx5xBhFCY@google.com>
Date: Fri, 16 Aug 2024 18:00:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Matlack <dmatlack@...gle.com>
Cc: 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 Rientjes <rientjes@...gle.com>, James Morse <james.morse@....com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, 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>, Yu Zhao <yuzhao@...gle.com>, 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 08/11] KVM: x86: Optimize kvm_{test_,}age_gfn a little bit

On Thu, Jul 25, 2024, David Matlack wrote:
> On 2024-07-24 01:10 AM, James Houghton wrote:
> > Optimize both kvm_age_gfn and kvm_test_age_gfn's interaction with the
> 
> nit: Use () when referring to functions.
> 
> > shadow MMU by, rather than checking if our memslot has rmaps, check if
> > there are any indirect_shadow_pages at all.
> 
> What is optimized by checking indirect_shadow_pages instead of
> have_rmaps and what's the benefit? Smells like a premature optimization.

Checking indirect_shadow_pages avoids taking mmu_lock for write when KVM doesn't
currently have shadow MMU pages, but did at some point in the past, whereas
kvm_memslots_have_rmaps() is sticky and will return true forever.

> > Also, for kvm_test_age_gfn, reorder the TDP MMU check to be first. If we
> > find that the range is young, we do not need to check the shadow MMU.
> 
> This should be a separate commit since it's a logically distinct change
> and no dependency on the other change in this commit (other than both
> touch the same function).
> 
> Splitting the commits up will also make it easier to write more specific
> short logs (instead of "optimize a little bit" :)

+1.  Especially code movement and refactoring, e.g. factoring out
tdp_mmu_clear_spte_bits_atomic() would ideally be in a standalone patch that's
dead simple to review.