Re: [PATCH v3 03/12] KVM: arm64: Add helper for creating unlinked stage2 subtrees

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Feb 15, 2023 at 05:40:37PM +0000, Ricardo Koller wrote:
> Add a stage2 helper, kvm_pgtable_stage2_create_unlinked(), for creating
> unlinked tables (the opposite of kvm_pgtable_stage2_free_unlinked()).

You don't need to mention the free_unlinked() side of things, IMO.

> Creating an unlinked table is useful for splitting block PTEs into
> subtrees of 4K PTEs.  For example, a 1G block PTE can be split into 4K
> PTEs by first creating a fully populated tree, and then use it to
> replace the 1G PTE in a single step.  This will be used in a
> subsequent commit for eager huge-page splitting (a dirty-logging
> optimization).

This is all very PAGE_SIZE=4K centric :) Could you instead describe the
operation in terms of the Linux page table hierarchy?

i.e. '... useful for splitting hugepages into PTE-level mappings'

> No functional change intended. This new function will be used in a
> subsequent commit.
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 29 +++++++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 47 ++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 7c45082e6c23..2ea397ad3e63 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -460,6 +460,35 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt);
>   */
>  void kvm_pgtable_stage2_free_unlinked(struct kvm_pgtable_mm_ops *mm_ops, void *pgtable, u32 level);
>  
> +/**
> + * kvm_pgtable_stage2_create_unlinked() - Create an unlinked stage-2 paging structure.
> + * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init*().
> + * @new:	Unlinked stage-2 paging structure to be created.
> + * @phys:	Physical address of the memory to map.
> + * @level:	Level of the stage-2 paging structure to be created.

I believe this is the starting level of the page table to be created,
right? Might be worth calling that out explicitly as level could also be
interpreted as mapping level.

> + * @prot:	Permissions and attributes for the mapping.
> + * @mc:		Cache of pre-allocated and zeroed memory from which to allocate
> + *		page-table pages.
> + * @force_pte:  Force mappings to PAGE_SIZE granularity.
> + *
> + * Create an unlinked page-table tree under @new. If @force_pte is
> + * true or @level is the PMD level, then the tree is mapped up to the
> + * PAGE_SIZE leaf PTE; the tree is mapped up one level otherwise. This
> + * new page-table tree is not reachable (i.e., it is removed) from the

'i.e. it is unlinked'

> + * root pgd and it's therefore unreachableby the hardware page-table
> + * walker. No TLB invalidation or CMOs are performed.
> + *
> + * If device attributes are not explicitly requested in @prot, then the
> + * mapping will be normal, cacheable.
> + *
> + * Return: 0 only if a fully populated tree was created (all memory
> + * under @level is mapped), negative error code on failure.
> + */
> +int kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> +				       kvm_pte_t *new, u64 phys, u32 level,
> +				       enum kvm_pgtable_prot prot, void *mc,
> +				       bool force_pte);
> +
>  /**
>   * kvm_pgtable_stage2_map() - Install a mapping in a guest stage-2 page-table.
>   * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init*().
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 0a5ef9288371..fed314f2b320 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1181,6 +1181,53 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
>  	return kvm_pgtable_walk(pgt, addr, size, &walker);
>  }
>  
> +int kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> +				      kvm_pte_t *new, u64 phys, u32 level,
> +				      enum kvm_pgtable_prot prot, void *mc,
> +				      bool force_pte)
> +{
> +	struct stage2_map_data map_data = {
> +		.phys		= phys,
> +		.mmu		= pgt->mmu,
> +		.memcache	= mc,
> +		.force_pte	= force_pte,
> +	};
> +	struct kvm_pgtable_walker walker = {
> +		.cb		= stage2_map_walker,
> +		.flags		= KVM_PGTABLE_WALK_LEAF |
> +				  KVM_PGTABLE_WALK_SKIP_BBM |
> +				  KVM_PGTABLE_WALK_SKIP_CMO,
> +		.arg		= &map_data,
> +	};
> +	/* .addr (the IPA) is irrelevant for a removed table */
> +	struct kvm_pgtable_walk_data data = {
> +		.walker	= &walker,
> +		.addr	= 0,
> +		.end	= kvm_granule_size(level),
> +	};
> +	struct kvm_pgtable_mm_ops *mm_ops = pgt->mm_ops;
> +	kvm_pte_t *pgtable;
> +	int ret;
> +
> +	ret = stage2_set_prot_attr(pgt, prot, &map_data.attr);
> +	if (ret)
> +		return ret;
> +
> +	pgtable = mm_ops->zalloc_page(mc);
> +	if (!pgtable)
> +		return -ENOMEM;
> +
> +	ret = __kvm_pgtable_walk(&data, mm_ops, (kvm_pteref_t)pgtable,
> +				 level + 1);
> +	if (ret) {
> +		kvm_pgtable_stage2_free_unlinked(mm_ops, pgtable, level);
> +		mm_ops->put_page(pgtable);
> +		return ret;
> +	}
> +
> +	*new = kvm_init_table_pte(pgtable, mm_ops);
> +	return 0;

Hmm. I don't think you really need the 'out' pointer here. Either the
function fails and it returns a negative error, or it succeeds and
returns a pointer.

  kvm_pte_t *kvm_pgtable_stage2_create_unlinked(...)
  {
          int ret;

	  [...]

	  ret = __kvm_pgtable_walk(...);
	  if (ret) {
	          kvm_pgtable_stage2_free_unlinked(...);
		  mm_ops->put_page(pgtable);
		  return ERR_PTR(ret);
	  }


          return pgtable;
  }

  [...]

  pgtable = kvm_pgtable_stage2_create_unlinked(...);
  if (IS_ERR(pgtable))
  	return PTR_ERR(pgtable);

-- 
Thanks,
Oliver