Re: [RFC PATCH 02/12] KVM: arm64: Allow visiting block PTEs in post-order

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On Sat, Nov 12, 2022 at 08:17:04AM +0000, Ricardo Koller wrote:
> The page table walker does not visit block PTEs in post-order. But there
> are some cases where doing so would be beneficial, for example: breaking a
> 1G block PTE into a full tree in post-order avoids visiting the new tree.
> 
> Allow post order visits of block PTEs. This will be used in a subsequent
> commit for eagerly breaking huge pages.
> 
> Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  4 ++--
>  arch/arm64/kvm/hyp/nvhe/setup.c      |  2 +-
>  arch/arm64/kvm/hyp/pgtable.c         | 25 ++++++++++++-------------
>  3 files changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index e2edeed462e8..d2e4a5032146 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -255,7 +255,7 @@ struct kvm_pgtable {
>   *					entries.
>   * @KVM_PGTABLE_WALK_TABLE_PRE:		Visit table entries before their
>   *					children.
> - * @KVM_PGTABLE_WALK_TABLE_POST:	Visit table entries after their
> + * @KVM_PGTABLE_WALK_POST:		Visit leaf or table entries after their
>   *					children.

It is not immediately obvious from this change alone that promoting the
post-order traversal of every walker to cover leaf + table PTEs is safe.

Have you considered using a flag for just leaf post-order visits?

--
Thanks,
Oliver



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