On Tue, 07 Mar 2023 03:45:47 +0000,
Ricardo Koller <ricarkol@xxxxxxxxxx> wrote:
>
> Add a new stage2 function, kvm_pgtable_stage2_split(), for splitting a
> range of huge pages. This will be used for eager-splitting huge pages
> into PAGE_SIZE pages. The goal is to avoid having to split huge pages
> on write-protection faults, and instead use this function to do it
> ahead of time for large ranges (e.g., all guest memory in 1G chunks at
> a time).
>
> No functional change intended. This new function will be used in a
> subsequent commit.
Same comment as before about the usefulness of this last sentence.
>
> Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
> Reviewed-by: Shaoqin Huang <shahuang@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/kvm_pgtable.h | 30 +++++++
> arch/arm64/kvm/hyp/pgtable.c | 113 +++++++++++++++++++++++++++
> 2 files changed, 143 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index b7b3fc0fa7a5..40e323a718fc 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -665,6 +665,36 @@ bool kvm_pgtable_stage2_is_young(struct kvm_pgtable *pgt, u64 addr);
> */
> int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size);
>
> +/**
> + * kvm_pgtable_stage2_split() - Split a range of huge pages into leaf PTEs pointing
> + * to PAGE_SIZE guest pages.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical address from which to split.
> + * @size: Size of the range.
> + * @mc: Cache of pre-allocated and zeroed memory from which to allocate
> + * page-table pages.
> + * @mc_capacity: Number of pages in @mc.
> + *
> + * @addr and the end (@addr + @size) are effectively aligned down and up to
> + * the top level huge-page block size. This is an example using 1GB
> + * huge-pages and 4KB granules.
> + *
> + * [---input range---]
> + * : :
> + * [--1G block pte--][--1G block pte--][--1G block pte--][--1G block pte--]
> + * : :
> + * [--2MB--][--2MB--][--2MB--][--2MB--]
> + * : :
> + * [ ][ ][:][ ][ ][ ][ ][ ][:][ ][ ][ ]
> + * : :
So here, what alignment do we effectively get?
> + *
> + * Return: 0 on success, negative error code on failure. Note that
> + * kvm_pgtable_stage2_split() is best effort: it tries to break as many
> + * blocks in the input range as allowed by @mc_capacity.
> + */
> +int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size,
> + void *mc, u64 mc_capacity);
> +
> /**
> * kvm_pgtable_walk() - Walk a page-table.
> * @pgt: Page-table structure initialised by kvm_pgtable_*_init().
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 6bdfcb671b32..3149b98d1701 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1259,6 +1259,119 @@ kvm_pte_t *kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> return pgtable;
> }
>
> +struct stage2_split_data {
> + struct kvm_s2_mmu *mmu;
> + void *memcache;
> + u64 mc_capacity;
Why isn't this a pointer to a *real* memcache structure?
> +};
> +
> +/*
> + * Get the number of page-tables needed to replace a block with a
> + * fully populated tree, up to the PTE level, at particular level.
> + */
> +static inline int stage2_block_get_nr_page_tables(u32 level)
Please drop the inline. The compiler will figure it out.
> +{
> + if (WARN_ON_ONCE(level < KVM_PGTABLE_MIN_BLOCK_LEVEL ||
> + level >= KVM_PGTABLE_MAX_LEVELS))
> + return -EINVAL;
Move this check to the 'default' case below.
> +
> + switch (level) {
> + case 1:
> + return PTRS_PER_PTE + 1;
> + case 2:
> + return 1;
This is odd. Replacing a block by a table always requires
'PTRS_PER_PTE + 1' pages. Why 1? If this is some special treatment for
level-2 mappings, please spell it out.
> + case 3:
> + return 0;
> + default:
> + return -EINVAL;
> + };
> +}
> +
> +static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx,
> + enum kvm_pgtable_walk_flags visit)
> +{
> + struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> + struct stage2_split_data *data = ctx->arg;
> + kvm_pte_t pte = ctx->old, new, *childp;
> + enum kvm_pgtable_prot prot;
> + void *mc = data->memcache;
> + u32 level = ctx->level;
> + bool force_pte;
> + int nr_pages;
> + u64 phys;
> +
> + /* No huge-pages exist at the last level */
> + if (level == KVM_PGTABLE_MAX_LEVELS - 1)
> + return 0;
Why the check for level 3 in the previous function if never get there?
> +
> + /* We only split valid block mappings */
> + if (!kvm_pte_valid(pte))
> + return 0;
> +
> + nr_pages = stage2_block_get_nr_page_tables(level);
> + if (nr_pages < 0)
> + return nr_pages;
> +
> + if (data->mc_capacity >= nr_pages) {
> + /* Build a tree mapped down to the PTE granularity. */
> + force_pte = true;
> + } else {
> + /*
> + * Don't force PTEs. This requires a single page of PMDs at the
> + * PUD level, or a single page of PTEs at the PMD level. If we
> + * are at the PUD level, the PTEs will be created recursively.
> + */
I don't understand how you reach this 'single page' conclusion. You
need to explain why you get there.
> + force_pte = false;
> + nr_pages = 1;
> + }
> +
> + if (data->mc_capacity < nr_pages)
> + return -ENOMEM;
> +
> + phys = kvm_pte_to_phys(pte);
> + prot = kvm_pgtable_stage2_pte_prot(pte);
> +
> + childp = kvm_pgtable_stage2_create_unlinked(data->mmu->pgt, phys,
> + level, prot, mc, force_pte);
> + if (IS_ERR(childp))
> + return PTR_ERR(childp);
> +
> + if (!stage2_try_break_pte(ctx, data->mmu)) {
> + kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level);
> + mm_ops->put_page(childp);
> + return -EAGAIN;
> + }
> +
> + /*
> + * Note, the contents of the page table are guaranteed to be made
> + * visible before the new PTE is assigned because stage2_make_pte()
> + * writes the PTE using smp_store_release().
> + */
> + new = kvm_init_table_pte(childp, mm_ops);
> + stage2_make_pte(ctx, new);
> + dsb(ishst);
> + data->mc_capacity -= nr_pages;
> + return 0;
> +}
> +
> +int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size,
> + void *mc, u64 mc_capacity)
> +{
> + struct stage2_split_data split_data = {
> + .mmu = pgt->mmu,
> + .memcache = mc,
> + .mc_capacity = mc_capacity,
> + };
> +
> + struct kvm_pgtable_walker walker = {
> + .cb = stage2_split_walker,
> + .flags = KVM_PGTABLE_WALK_LEAF,
> + .arg = &split_data,
> + };
> +
> + return kvm_pgtable_walk(pgt, addr, size, &walker);
> +}
> +
> int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
> struct kvm_pgtable_mm_ops *mm_ops,
> enum kvm_pgtable_stage2_flags flags,
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
