Re: [RFC PATCH] KVM: x86: Support write protect huge pages lazily

[Keqian Zhu]

Hi Paolo,

I plan to rework this patch and do full test. What do you think about this idea
(enable dirty logging for huge pages lazily)?

Best Regards,
Keqian

PS: As dirty log of TDP MMU has been supported, I should add more code.

On 2020/8/28 16:11, Keqian Zhu wrote:
> Currently during enable dirty logging, if we're with init-all-set,
> we just write protect huge pages and leave normal pages untouched,
> for that we can enable dirty logging for these pages lazily.
> 
> It seems that enable dirty logging lazily for huge pages is feasible
> too, which not only reduces the time of start dirty logging, also
> greatly reduces side-effect on guest when there is high dirty rate.
> 
> (These codes are not tested, for RFC purpose :-) ).
> 
> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 +-
>  arch/x86/kvm/mmu/mmu.c          | 65 ++++++++++++++++++++++++++-------
>  arch/x86/kvm/vmx/vmx.c          |  3 +-
>  arch/x86/kvm/x86.c              | 22 +++++------
>  4 files changed, 62 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 5303dbc5c9bc..201a068cf43d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1296,8 +1296,7 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
>  
>  void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
>  void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
> -				      struct kvm_memory_slot *memslot,
> -				      int start_level);
> +				      struct kvm_memory_slot *memslot);
>  void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  				   const struct kvm_memory_slot *memslot);
>  void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 43fdb0c12a5d..4b7d577de6cd 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1625,14 +1625,45 @@ static bool __rmap_set_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head)
>  }
>  
>  /**
> - * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
> + * kvm_mmu_write_protect_largepage_masked - write protect selected largepages
>   * @kvm: kvm instance
>   * @slot: slot to protect
>   * @gfn_offset: start of the BITS_PER_LONG pages we care about
>   * @mask: indicates which pages we should protect
>   *
> - * Used when we do not need to care about huge page mappings: e.g. during dirty
> - * logging we do not have any such mappings.
> + * @ret: true if all pages are write protected
> + */
> +static bool kvm_mmu_write_protect_largepage_masked(struct kvm *kvm,
> +				    struct kvm_memory_slot *slot,
> +				    gfn_t gfn_offset, unsigned long mask)
> +{
> +	struct kvm_rmap_head *rmap_head;
> +	bool protected, all_protected;
> +	gfn_t start_gfn = slot->base_gfn + gfn_offset;
> +	int i;
> +
> +	all_protected = true;
> +	while (mask) {
> +		protected = false;
> +		for (i = PG_LEVEL_2M; i <= KVM_MAX_HUGEPAGE_LEVEL; ++i) {
> +			rmap_head = __gfn_to_rmap(start_gfn + __ffs(mask), i, slot);
> +			protectd |= __rmap_write_protect(kvm, rmap_head, false);
> +		}
> +
> +		all_protected &= protectd;
> +		/* clear the first set bit */
> +		mask &= mask - 1;
> +	}
> +
> +	return all_protected;
> +}
> +
> +/**
> + * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
> + * @kvm: kvm instance
> + * @slot: slot to protect
> + * @gfn_offset: start of the BITS_PER_LONG pages we care about
> + * @mask: indicates which pages we should protect
>   */
>  static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
>  				     struct kvm_memory_slot *slot,
> @@ -1679,18 +1710,25 @@ EXPORT_SYMBOL_GPL(kvm_mmu_clear_dirty_pt_masked);
>  
>  /**
>   * kvm_arch_mmu_enable_log_dirty_pt_masked - enable dirty logging for selected
> - * PT level pages.
> - *
> - * It calls kvm_mmu_write_protect_pt_masked to write protect selected pages to
> - * enable dirty logging for them.
> - *
> - * Used when we do not need to care about huge page mappings: e.g. during dirty
> - * logging we do not have any such mappings.
> + * dirty pages.
>   */
>  void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>  				struct kvm_memory_slot *slot,
>  				gfn_t gfn_offset, unsigned long mask)
>  {
> +	/*
> +	 * If we're with initial-all-set, huge pages are NOT
> +	 * write protected when we start dirty log, so we must
> +	 * write protect them here.
> +	 */
> +	if (kvm_dirty_log_manual_protect_and_init_set(kvm)) {
> +		if (kvm_mmu_write_protect_largepage_masked(kvm, slot,
> +					gfn_offset, mask))
> +			return;
> +	}
> +
> +	/* Then we can handle the 4K level pages */
> +
>  	if (kvm_x86_ops.enable_log_dirty_pt_masked)
>  		kvm_x86_ops.enable_log_dirty_pt_masked(kvm, slot, gfn_offset,
>  				mask);
> @@ -5906,14 +5944,13 @@ static bool slot_rmap_write_protect(struct kvm *kvm,
>  }
>  
>  void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
> -				      struct kvm_memory_slot *memslot,
> -				      int start_level)
> +				      struct kvm_memory_slot *memslot)
>  {
>  	bool flush;
>  
>  	spin_lock(&kvm->mmu_lock);
> -	flush = slot_handle_level(kvm, memslot, slot_rmap_write_protect,
> -				start_level, KVM_MAX_HUGEPAGE_LEVEL, false);
> +	flush = slot_handle_all_level(kvm, memslot, slot_rmap_write_protect,
> +				      false);
>  	spin_unlock(&kvm->mmu_lock);
>  
>  	/*
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 819c185adf09..ba871c52ef8b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7538,8 +7538,7 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu, int cpu)
>  static void vmx_slot_enable_log_dirty(struct kvm *kvm,
>  				     struct kvm_memory_slot *slot)
>  {
> -	if (!kvm_dirty_log_manual_protect_and_init_set(kvm))
> -		kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
> +	kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
>  	kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
>  }
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d39d6cf1d473..c31c32f1424b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10225,22 +10225,18 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
>  	 * is enabled the D-bit or the W-bit will be cleared.
>  	 */
>  	if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
> +		/*
> +		 * If we're with initial-all-set, we don't need
> +		 * to write protect any page because they're
> +		 * reported as dirty already.
> +		 */
> +		if (kvm_dirty_log_manual_protect_and_init_set(kvm))
> +			return;
> +
>  		if (kvm_x86_ops.slot_enable_log_dirty) {
>  			kvm_x86_ops.slot_enable_log_dirty(kvm, new);
>  		} else {
> -			int level =
> -				kvm_dirty_log_manual_protect_and_init_set(kvm) ?
> -				PG_LEVEL_2M : PG_LEVEL_4K;
> -
> -			/*
> -			 * If we're with initial-all-set, we don't need
> -			 * to write protect any small page because
> -			 * they're reported as dirty already.  However
> -			 * we still need to write-protect huge pages
> -			 * so that the page split can happen lazily on
> -			 * the first write to the huge page.
> -			 */
> -			kvm_mmu_slot_remove_write_access(kvm, new, level);
> +			kvm_mmu_slot_remove_write_access(kvm, new);
>  		}
>  	} else {
>  		if (kvm_x86_ops.slot_disable_log_dirty)
>