linux-kernel - Re: [Patch v3 4/7] KVM: x86/mmu: Optimize SPTE change for aging gfn range

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Message-ID: <Y+1LYdjXu4H6PX1L@google.com>
Date:   Wed, 15 Feb 2023 13:15:13 -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 v3 4/7] KVM: x86/mmu: Optimize SPTE change for aging gfn
 range

On Fri, Feb 10, 2023 at 05:46:23PM -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.

nit: State what the patch does first.

> 
> 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.
> 
> Signed-off-by: Vipin Sharma <vipinsh@...gle.com>

nits aside,

Reviewed-by: David Matlack <dmatlack@...gle.com>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 36 +++++++++++++++++++-----------------
>  1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index c895560244de..5d6e77554797 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -758,13 +758,6 @@ static inline void tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
>  	_tdp_mmu_set_spte(kvm, iter, new_spte, true);
>  }
>  
> -static inline void tdp_mmu_set_spte_no_acc_track(struct kvm *kvm,
> -						 struct tdp_iter *iter,
> -						 u64 new_spte)
> -{
> -	_tdp_mmu_set_spte(kvm, iter, new_spte, false);
> -}
> -
>  #define tdp_root_for_each_pte(_iter, _root, _start, _end) \
>  	for_each_tdp_pte(_iter, _root, _start, _end)
>  
> @@ -1251,32 +1244,41 @@ 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
> + * the clear_young() or clear_flush_young() notifier, which uses the return
> + * value to determine if the page has been accessed.
>   */
>  static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
>  			  struct kvm_gfn_range *range)
>  {
> -	u64 new_spte = 0;
> +	u64 new_spte;
>  
>  	/* If we have a non-accessed entry we don't need to change the pte. */
>  	if (!is_accessed_spte(iter->old_spte))
>  		return false;
>  
> -	new_spte = iter->old_spte;
> -
> -	if (spte_ad_enabled(new_spte)) {
> -		new_spte &= ~shadow_accessed_mask;
> +	if (spte_ad_enabled(iter->old_spte)) {
> +		iter->old_spte  = tdp_mmu_clear_spte_bits(iter->sptep,

nit: Extra space before =