Re: [PATCH v5 11/14] KVM: arm64: Make block->table PTE changes parallel-aware

[Ben Gardon <bgardon@google.com>]

On Mon, Nov 7, 2022 at 1:59 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> In order to service stage-2 faults in parallel, stage-2 table walkers
> must take exclusive ownership of the PTE being worked on. An additional
> requirement of the architecture is that software must perform a
> 'break-before-make' operation when changing the block size used for
> mapping memory.
>
> Roll these two concepts together into helpers for performing a
> 'break-before-make' sequence. Use a special PTE value to indicate a PTE
> has been locked by a software walker. Additionally, use an atomic
> compare-exchange to 'break' the PTE when the stage-2 page tables are
> possibly shared with another software walker. Elide the DSB + TLBI if
> the evicted PTE was invalid (and thus not subject to break-before-make).
>
> All of the atomics do nothing for now, as the stage-2 walker isn't fully
> ready to perform parallel walks.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 80 +++++++++++++++++++++++++++++++++---
>  1 file changed, 75 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index f4dd77c6c97d..b9f0d792b8d9 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -49,6 +49,12 @@
>  #define KVM_INVALID_PTE_OWNER_MASK     GENMASK(9, 2)
>  #define KVM_MAX_OWNER_ID               1
>
> +/*
> + * Used to indicate a pte for which a 'break-before-make' sequence is in
> + * progress.
> + */
> +#define KVM_INVALID_PTE_LOCKED         BIT(10)
> +
>  struct kvm_pgtable_walk_data {
>         struct kvm_pgtable_walker       *walker;
>
> @@ -674,6 +680,11 @@ static bool stage2_pte_is_counted(kvm_pte_t pte)
>         return !!pte;
>  }
>
> +static bool stage2_pte_is_locked(kvm_pte_t pte)
> +{
> +       return !kvm_pte_valid(pte) && (pte & KVM_INVALID_PTE_LOCKED);
> +}
> +
>  static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new)
>  {
>         if (!kvm_pgtable_walk_shared(ctx)) {
> @@ -684,6 +695,64 @@ static bool stage2_try_set_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_
>         return cmpxchg(ctx->ptep, ctx->old, new) == ctx->old;
>  }
>
> +/**
> + * stage2_try_break_pte() - Invalidates a pte according to the
> + *                         'break-before-make' requirements of the
> + *                         architecture.
> + *
> + * @ctx: context of the visited pte.
> + * @mmu: stage-2 mmu
> + *
> + * Returns: true if the pte was successfully broken.
> + *
> + * If the removed pte was valid, performs the necessary serialization and TLB
> + * invalidation for the old value. For counted ptes, drops the reference count
> + * on the containing table page.
> + */
> +static bool stage2_try_break_pte(const struct kvm_pgtable_visit_ctx *ctx,
> +                                struct kvm_s2_mmu *mmu)
> +{
> +       struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> +
> +       if (stage2_pte_is_locked(ctx->old)) {
> +               /*
> +                * Should never occur if this walker has exclusive access to the
> +                * page tables.
> +                */
> +               WARN_ON(!kvm_pgtable_walk_shared(ctx));
> +               return false;
> +       }
> +
> +       if (!stage2_try_set_pte(ctx, KVM_INVALID_PTE_LOCKED))
> +               return false;
> +
> +       /*
> +        * Perform the appropriate TLB invalidation based on the evicted pte
> +        * value (if any).
> +        */
> +       if (kvm_pte_table(ctx->old, ctx->level))
> +               kvm_call_hyp(__kvm_tlb_flush_vmid, mmu);
> +       else if (kvm_pte_valid(ctx->old))
> +               kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu, ctx->addr, ctx->level);
> +
> +       if (stage2_pte_is_counted(ctx->old))
> +               mm_ops->put_page(ctx->ptep);
> +
> +       return true;
> +}
> +
> +static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new)
> +{
> +       struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
> +
> +       WARN_ON(!stage2_pte_is_locked(*ctx->ptep));
> +
> +       if (stage2_pte_is_counted(new))
> +               mm_ops->get_page(ctx->ptep);
> +
> +       smp_store_release(ctx->ptep, new);
> +}
> +
>  static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu,
>                            struct kvm_pgtable_mm_ops *mm_ops)
>  {
> @@ -812,17 +881,18 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
>         if (!childp)
>                 return -ENOMEM;
>
> +       if (!stage2_try_break_pte(ctx, data->mmu)) {
> +               mm_ops->put_page(childp);
> +               return -EAGAIN;
> +       }
> +
>         /*
>          * If we've run into an existing block mapping then replace it with
>          * a table. Accesses beyond 'end' that fall within the new table
>          * will be mapped lazily.
>          */
> -       if (stage2_pte_is_counted(ctx->old))
> -               stage2_put_pte(ctx, data->mmu, mm_ops);
> -
>         new = kvm_init_table_pte(childp, mm_ops);

Does it make any sense to move this before the "break" to minimize the
critical section in which the PTE is locked?


> -       mm_ops->get_page(ctx->ptep);
> -       smp_store_release(ctx->ptep, new);
> +       stage2_make_pte(ctx, new);
>
>         return 0;
>  }
> --
> 2.38.1.431.g37b22c650d-goog
>