linux-kernel - Re: [Patch v2 3/5] KVM: x86/mmu: Optimize SPTE change for aging gfn range

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Message-ID: <Y+GTyVHvpskr8YxD@google.com>
Date:   Mon, 6 Feb 2023 15:56:57 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, bgardon@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v2 3/5] KVM: x86/mmu: Optimize SPTE change for aging gfn
 range

On Fri, Feb 03, 2023 at 11:28:20AM -0800, Vipin Sharma wrote:
> No need to check all of the conditions in __handle_changed_spte(). Aging
> a gfn range implies resetting access bit or marking spte for access
> tracking.
> 
> Use atomic operation to only reset those bits. This avoids checking many
> conditions in __handle_changed_spte() API. Also, clean up code by
> removing dead code and API parameters.

Suggest splitting out the dead code cleanup to make it easier to review.

> 
> Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 68 ++++++++++++++------------------------
>  1 file changed, 25 insertions(+), 43 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 83f15052aa6c..18630a06fa1f 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1251,32 +1228,37 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
>  /*
>   * Mark the SPTEs range of GFNs [start, end) unaccessed and return non-zero
>   * if any of the GFNs in the range have been accessed.
> + *
> + * No need to mark corresponding PFN as accessed as this call is coming from
> + * MMU notifier for that page via HVA.

Thanks for adding this comment.

Can you just extend it to mention that the information is passed via the
return value? e.g.

 * No need to mark corresponding PFN as accessed as this call is coming
 * from the clear_young() or clear_flush_young() notifier, which uses
 * the return value to determine if the page has been accessed.