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.up...@linux.dev>
> ---
> 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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