Re: [RFC 05/19] KVM: x86/mmu: Remove redundant flushes when disabling dirty logging

[David Matlack]

On Wed, Nov 10, 2021 at 02:29:56PM -0800, Ben Gardon wrote:
> tdp_mmu_zap_spte_atomic flushes on every zap already, so no need to
> flush again after it's done.
> 
> 
> Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>

Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>

> ---
>  arch/x86/kvm/mmu/mmu.c     |  4 +---
>  arch/x86/kvm/mmu/tdp_mmu.c | 21 ++++++---------------
>  arch/x86/kvm/mmu/tdp_mmu.h |  5 ++---
>  3 files changed, 9 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 354d2ca92df4..baa94acab516 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5870,9 +5870,7 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  
>  	if (is_tdp_mmu_enabled(kvm)) {
>  		read_lock(&kvm->mmu_lock);
> -		flush = kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot, flush);
> -		if (flush)
> -			kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
> +		kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot);
>  		read_unlock(&kvm->mmu_lock);
>  	}
>  }
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index c2a9f7acf8ef..1ece645e737f 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1438,10 +1438,9 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
>   * Clear leaf entries which could be replaced by large mappings, for
>   * GFNs within the slot.
>   */
> -static bool zap_collapsible_spte_range(struct kvm *kvm,
> +static void zap_collapsible_spte_range(struct kvm *kvm,
>  				       struct kvm_mmu_page *root,
> -				       const struct kvm_memory_slot *slot,
> -				       bool flush)
> +				       const struct kvm_memory_slot *slot)
>  {
>  	gfn_t start = slot->base_gfn;
>  	gfn_t end = start + slot->npages;
> @@ -1452,10 +1451,8 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,
>  
>  	tdp_root_for_each_pte(iter, root, start, end) {
>  retry:
> -		if (tdp_mmu_iter_cond_resched(kvm, &iter, flush, true)) {
> -			flush = false;
> +		if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
>  			continue;
> -		}
>  
>  		if (!is_shadow_present_pte(iter.old_spte) ||
>  		    !is_last_spte(iter.old_spte, iter.level))
> @@ -1475,30 +1472,24 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,
>  			iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
>  			goto retry;
>  		}
> -		flush = true;
>  	}
>  
>  	rcu_read_unlock();
> -
> -	return flush;
>  }
>  
>  /*
>   * Clear non-leaf entries (and free associated page tables) which could
>   * be replaced by large mappings, for GFNs within the slot.
>   */
> -bool kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> -				       const struct kvm_memory_slot *slot,
> -				       bool flush)
> +void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> +				       const struct kvm_memory_slot *slot)
>  {
>  	struct kvm_mmu_page *root;
>  
>  	lockdep_assert_held_read(&kvm->mmu_lock);
>  
>  	for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, true)
> -		flush = zap_collapsible_spte_range(kvm, root, slot, flush);
> -
> -	return flush;
> +		zap_collapsible_spte_range(kvm, root, slot);
>  }
>  
>  /*
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index 476b133544dd..3899004a5d91 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -64,9 +64,8 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
>  				       struct kvm_memory_slot *slot,
>  				       gfn_t gfn, unsigned long mask,
>  				       bool wrprot);
> -bool kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> -				       const struct kvm_memory_slot *slot,
> -				       bool flush);
> +void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> +				       const struct kvm_memory_slot *slot);
>  
>  bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
>  				   struct kvm_memory_slot *slot, gfn_t gfn,
> -- 
> 2.34.0.rc0.344.g81b53c2807-goog
>