linux-kernel - Re: [PATCH v7 04/18] KVM: x86/mmu: Relax locking for kvm_test_age_gfn and kvm_age

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Message-ID: <CADrL8HXZttWQcsUBPYBuctEP4BWhGwSoTNvT7eeknsGJkgbGig@mail.gmail.com>
Date: Wed, 25 Sep 2024 18:55:07 -0700
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, 
	Yu Zhao <yuzhao@...gle.com>, Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v7 04/18] KVM: x86/mmu: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Wed, Sep 25, 2024 at 6:35 PM James Houghton <jthoughton@...gle.com> wrote:
>
> Walk the TDP MMU in an RCU read-side critical section without holding
> mmu_lock when harvesting and potentially updating age information on
> sptes. This requires a way to do RCU-safe walking of the tdp_mmu_roots;
> do this with a new macro. The PTE modifications are now done atomically,
> and kvm_tdp_mmu_spte_need_atomic_write() has been updated to account for
> the fact that kvm_age_gfn can now lockless update the accessed bit and
> the W/R/X bits).
>
> If the cmpxchg for marking the spte for access tracking fails, leave it
> as is and treat it as if it were young, as if the spte is being actively
> modified, it is most likely young.
>
> Harvesting age information from the shadow MMU is still done while
> holding the MMU write lock.
>
> Suggested-by: Yu Zhao <yuzhao@...gle.com>
> Signed-off-by: James Houghton <jthoughton@...gle.com>

Oh no! I have left off David Matlack's Reviewed-bys[1, 2] from this
patch and from patch 2[3], and I failed to apply his comment
suggestion on this patch like I said I would. Sorry David, I have
fixed up my tree now.

[1]: https://lore.kernel.org/linux-mm/ZqJ_xANKf3bNcaHM@google.com/
[2]: https://lore.kernel.org/linux-mm/ZqKUefN3HgBQQkuA@google.com/
[3]: https://lore.kernel.org/kvm/20240926013506.860253-3-jthoughton@google.com/