Re: [PATCH v2 06/15] KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make

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On Fri, Oct 07, 2022 at 11:28:09PM +0000, Oliver Upton wrote:
> The break-before-make sequence is a bit annoying as it opens a window
> wherein memory is unmapped from the guest. KVM should replace the PTE
> as quickly as possible and avoid unnecessary work in between.
> 
> Presently, the stage-2 map walker tears down a removed table before
> installing a block mapping when coalescing a table into a block. As the
> removed table is no longer visible to hardware walkers after the
> DSB+TLBI, it is possible to move the remaining cleanup to happen after
> installing the new PTE.
> 
> Reshuffle the stage-2 map walker to install the new block entry in
> the pre-order callback. Unwire all of the teardown logic and replace
> it with a call to kvm_pgtable_stage2_free_removed() after fixing
> the PTE. The post-order visitor is now completely unnecessary, so drop
> it. Finally, touch up the comments to better represent the now
> simplified map walker.
> 
> Note that the call to tear down the unlinked stage-2 is indirected
> as a subsequent change will use an RCU callback to trigger tear down.
> RCU is not available to pKVM, so there is a need to use different
> implementations on pKVM and non-pKVM VMs.
> 
> Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h  |  3 +
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 ++
>  arch/arm64/kvm/hyp/pgtable.c          | 84 ++++++++-------------------
>  arch/arm64/kvm/mmu.c                  |  8 +++
>  4 files changed, 40 insertions(+), 61 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 71b7d154b78a..c33edcf36b5b 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -77,6 +77,8 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
>   *				allocation is physically contiguous.
>   * @free_pages_exact:		Free an exact number of memory pages previously
>   *				allocated by zalloc_pages_exact.
> + * @free_removed_table:		Free a removed paging structure by unlinking and
> + *				dropping references.
>   * @get_page:			Increment the refcount on a page.
>   * @put_page:			Decrement the refcount on a page. When the
>   *				refcount reaches 0 the page is automatically
> @@ -95,6 +97,7 @@ struct kvm_pgtable_mm_ops {
>  	void*		(*zalloc_page)(void *arg);
>  	void*		(*zalloc_pages_exact)(size_t size);
>  	void		(*free_pages_exact)(void *addr, size_t size);
> +	void		(*free_removed_table)(void *addr, u32 level);
>  	void		(*get_page)(void *addr);
>  	void		(*put_page)(void *addr);
>  	int		(*page_count)(void *addr);
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index d21d1b08a055..735769886b55 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -79,6 +79,11 @@ static void host_s2_put_page(void *addr)
>  	hyp_put_page(&host_s2_pool, addr);
>  }
>  
> +static void host_s2_free_removed_table(void *addr, u32 level)
> +{
> +	kvm_pgtable_stage2_free_removed(&host_kvm.mm_ops, addr, level);
> +}
> +
>  static int prepare_s2_pool(void *pgt_pool_base)
>  {
>  	unsigned long nr_pages, pfn;
> @@ -93,6 +98,7 @@ static int prepare_s2_pool(void *pgt_pool_base)
>  	host_kvm.mm_ops = (struct kvm_pgtable_mm_ops) {
>  		.zalloc_pages_exact = host_s2_zalloc_pages_exact,
>  		.zalloc_page = host_s2_zalloc_page,
> +		.free_removed_table = host_s2_free_removed_table,
>  		.phys_to_virt = hyp_phys_to_virt,
>  		.virt_to_phys = hyp_virt_to_phys,
>  		.page_count = hyp_page_count,
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 363a5cce7e1a..02c33fccb178 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -746,16 +746,19 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
>  	return 0;
>  }
>  
> +static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> +				struct stage2_map_data *data);
> +
>  static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx,
>  				     struct stage2_map_data *data)
>  {
> -	if (data->anchor)
> -		return 0;
> +	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> +	kvm_pte_t *childp = kvm_pte_follow(ctx->old, mm_ops);
> +	int ret;
>  
>  	if (!stage2_leaf_mapping_allowed(ctx, data))
>  		return 0;
>  
> -	data->childp = kvm_pte_follow(ctx->old, ctx->mm_ops);
>  	kvm_clear_pte(ctx->ptep);
>  
>  	/*
> @@ -764,8 +767,13 @@ static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx,
>  	 * individually.
>  	 */
>  	kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
> -	data->anchor = ctx->ptep;
> -	return 0;
> +
> +	ret = stage2_map_walk_leaf(ctx, data);
> +
> +	mm_ops->put_page(ctx->ptep);
> +	mm_ops->free_removed_table(childp, ctx->level + 1);

This could save some cycles:

	if (stage2_pte_is_counted(ctx->old)
		mm_ops->free_removed_table(childp, ctx->level + 1);

as coming back using the stage2_free_walker() requires some preparation,
and it does the exact same check anyawy.

> +
> +	return ret;
>  }
>  
>  static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> @@ -775,13 +783,6 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
>  	kvm_pte_t *childp;
>  	int ret;
>  
> -	if (data->anchor) {
> -		if (stage2_pte_is_counted(ctx->old))
> -			mm_ops->put_page(ctx->ptep);
> -
> -		return 0;
> -	}
> -
>  	ret = stage2_map_walker_try_leaf(ctx, data);
>  	if (ret != -E2BIG)
>  		return ret;
> @@ -810,49 +811,14 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
>  	return 0;
>  }
>  
> -static int stage2_map_walk_table_post(const struct kvm_pgtable_visit_ctx *ctx,
> -				      struct stage2_map_data *data)
> -{
> -	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> -	kvm_pte_t *childp;
> -	int ret = 0;
> -
> -	if (!data->anchor)
> -		return 0;
> -
> -	if (data->anchor == ctx->ptep) {
> -		childp = data->childp;
> -		data->anchor = NULL;
> -		data->childp = NULL;
> -		ret = stage2_map_walk_leaf(ctx, data);
> -	} else {
> -		childp = kvm_pte_follow(ctx->old, mm_ops);
> -	}
> -
> -	mm_ops->put_page(childp);
> -	mm_ops->put_page(ctx->ptep);
> -
> -	return ret;
> -}
> -
>  /*
> - * This is a little fiddly, as we use all three of the walk flags. The idea
> - * is that the TABLE_PRE callback runs for table entries on the way down,
> - * looking for table entries which we could conceivably replace with a
> - * block entry for this mapping. If it finds one, then it sets the 'anchor'
> - * field in 'struct stage2_map_data' to point at the table entry, before
> - * clearing the entry to zero and descending into the now detached table.
> - *
> - * The behaviour of the LEAF callback then depends on whether or not the
> - * anchor has been set. If not, then we're not using a block mapping higher
> - * up the table and we perform the mapping at the existing leaves instead.
> - * If, on the other hand, the anchor _is_ set, then we drop references to
> - * all valid leaves so that the pages beneath the anchor can be freed.
> + * The TABLE_PRE callback runs for table entries on the way down, looking
> + * for table entries which we could conceivably replace with a block entry
> + * for this mapping. If it finds one it replaces the entry and calls
> + * kvm_pgtable_mm_ops::free_removed_table() to tear down the detached table.
>   *
> - * Finally, the TABLE_POST callback does nothing if the anchor has not
> - * been set, but otherwise frees the page-table pages while walking back up
> - * the page-table, installing the block entry when it revisits the anchor
> - * pointer and clearing the anchor to NULL.
> + * Otherwise, the LEAF callback performs the mapping at the existing leaves
> + * instead.
>   */
>  static int stage2_map_walker(const struct kvm_pgtable_visit_ctx *ctx,
>  			     enum kvm_pgtable_walk_flags visit)
> @@ -864,11 +830,9 @@ static int stage2_map_walker(const struct kvm_pgtable_visit_ctx *ctx,
>  		return stage2_map_walk_table_pre(ctx, data);
>  	case KVM_PGTABLE_WALK_LEAF:
>  		return stage2_map_walk_leaf(ctx, data);
> -	case KVM_PGTABLE_WALK_TABLE_POST:
> -		return stage2_map_walk_table_post(ctx, data);
> +	default:
> +		return -EINVAL;
>  	}
> -
> -	return -EINVAL;
>  }
>  
>  int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> @@ -885,8 +849,7 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_map_walker,
>  		.flags		= KVM_PGTABLE_WALK_TABLE_PRE |
> -				  KVM_PGTABLE_WALK_LEAF |
> -				  KVM_PGTABLE_WALK_TABLE_POST,
> +				  KVM_PGTABLE_WALK_LEAF,
>  		.arg		= &map_data,
>  	};
>  
> @@ -916,8 +879,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_map_walker,
>  		.flags		= KVM_PGTABLE_WALK_TABLE_PRE |
> -				  KVM_PGTABLE_WALK_LEAF |
> -				  KVM_PGTABLE_WALK_TABLE_POST,
> +				  KVM_PGTABLE_WALK_LEAF,
>  		.arg		= &map_data,
>  	};
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c9a13e487187..04a25319abb0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -102,6 +102,13 @@ static void *kvm_host_zalloc_pages_exact(size_t size)
>  	return alloc_pages_exact(size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>  }
>  
> +static struct kvm_pgtable_mm_ops kvm_s2_mm_ops;
> +
> +static void stage2_free_removed_table(void *addr, u32 level)
> +{
> +	kvm_pgtable_stage2_free_removed(&kvm_s2_mm_ops, addr, level);
> +}
> +
>  static void kvm_host_get_page(void *addr)
>  {
>  	get_page(virt_to_page(addr));
> @@ -627,6 +634,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
>  	.zalloc_page		= stage2_memcache_zalloc_page,
>  	.zalloc_pages_exact	= kvm_host_zalloc_pages_exact,
>  	.free_pages_exact	= free_pages_exact,
> +	.free_removed_table	= stage2_free_removed_table,
>  	.get_page		= kvm_host_get_page,
>  	.put_page		= kvm_host_put_page,
>  	.page_count		= kvm_host_page_count,
> -- 
> 2.38.0.rc1.362.ged0d419d3c-goog
> 



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