Re: [PATCH v6 11/12] KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG

[Marc Zyngier <maz@kernel.org>]

On Tue, 07 Mar 2023 03:45:54 +0000,
Ricardo Koller <ricarkol@google.com> wrote:
> 
> This is the arm64 counterpart of commit cb00a70bd4b7 ("KVM: x86/mmu:
> Split huge pages mapped by the TDP MMU during KVM_CLEAR_DIRTY_LOG"),
> which has the benefit of splitting the cost of splitting a memslot
> across multiple ioctls.
> 
> Split huge pages on the range specified using KVM_CLEAR_DIRTY_LOG.
> And do not split when enabling dirty logging if
> KVM_DIRTY_LOG_INITIALLY_SET is set.
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>  arch/arm64/kvm/mmu.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 910aea6bbd1e..d54223b5db97 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1089,8 +1089,8 @@ static void kvm_mmu_split_memory_region(struct kvm *kvm, int slot)
>   * @mask:	The mask of pages at offset 'gfn_offset' in this memory
>   *		slot to enable dirty logging on
>   *
> - * Writes protect selected pages to enable dirty logging for them. Caller must
> - * acquire kvm->mmu_lock.
> + * Splits selected pages to PAGE_SIZE and then writes protect them to enable
> + * dirty logging for them. Caller must acquire kvm->mmu_lock.

The code does things in the opposite order...

>   */
>  void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>  		struct kvm_memory_slot *slot,
> @@ -1103,6 +1103,13 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>  	lockdep_assert_held_write(&kvm->mmu_lock);
>  
>  	stage2_wp_range(&kvm->arch.mmu, start, end);
> +
> +	/*
> +	 * If initially-all-set mode is not set, then huge-pages were already
> +	 * split when enabling dirty logging: no need to do it again.
> +	 */
> +	if (kvm_dirty_log_manual_protect_and_init_set(kvm))

This contradicts the comment. Which one is correct?

> +		kvm_mmu_split_huge_pages(kvm, start, end);
>  }
>  
>  static void kvm_send_hwpoison_signal(unsigned long address, short lsb)
> @@ -1889,7 +1896,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  		 * this when deleting, moving, disabling dirty logging, or
>  		 * creating the memslot (a nop). Doing it for deletes makes
>  		 * sure we don't leak memory, and there's no need to keep the
> -		 * cache around for any of the other cases.
> +		 * cache around for any of the other cases. Keeping the cache
> +		 * is useful for successive KVM_CLEAR_DIRTY_LOG calls, which is
> +		 * not handled in this function.

Where is it handled then?

>  		 */
>  		kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
>  	}

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.