Re: [PATCH 2/9] KVM: arm64: Add helper for creating removed stage2 subtrees

[Ricardo Koller <ricarkol@google.com>]

On Mon, Jan 23, 2023 at 04:55:40PM -0800, Ben Gardon wrote:
> On Thu, Jan 12, 2023 at 7:50 PM Ricardo Koller <ricarkol@google.com> wrote:
> >
> > Add a stage2 helper, kvm_pgtable_stage2_create_removed(), for creating
> > removed tables (the opposite of kvm_pgtable_stage2_free_removed()).
> > Creating a removed 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).
> >
> > 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 | 25 +++++++++++++++
> >  arch/arm64/kvm/hyp/pgtable.c         | 47 ++++++++++++++++++++++++++++
> >  2 files changed, 72 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 84a271647007..8ad78d61af7f 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -450,6 +450,31 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt);
> >   */
> >  void kvm_pgtable_stage2_free_removed(struct kvm_pgtable_mm_ops *mm_ops, void *pgtable, u32 level);
> >
> > +/**
> > + * kvm_pgtable_stage2_free_removed() - Create a removed stage-2 paging structure.
> > + * @pgt:       Page-table structure initialised by kvm_pgtable_stage2_init*().
> > + * @new:       Unlinked stage-2 paging structure to be created.
> 
> Oh, I see so the "removed" page table is actually a new page table
> that has never been part of the paging structure. In that case I would
> find it much more intuitive to call it "unlinked" or similar.
>

Sounds good, I like "unlinked".

Oliver, are you OK if I rename free_removed() as well? just to keep them
symmetric.

> > + * @phys:      Physical address of the memory to map.
> > + * @level:     Level of the stage-2 paging structure to be created.
> > + * @prot:      Permissions and attributes for the mapping.
> > + * @mc:                Cache of pre-allocated and zeroed memory from which to allocate
> > + *             page-table pages.
> > + *
> > + * Create a removed page-table tree of PAGE_SIZE leaf PTEs under *new.
> > + * This new page-table tree is not reachable (i.e., it is removed) from the
> > + * 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, negative error
> > + * code on failure. No partially-populated table can be returned.
> > + */
> > +int kvm_pgtable_stage2_create_removed(struct kvm_pgtable *pgt,
> > +                                     kvm_pte_t *new, u64 phys, u32 level,
> > +                                     enum kvm_pgtable_prot prot, void *mc);
> > +
> >  /**
> >   * 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 87fd40d09056..0dee13007776 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);
> >  }
> >
> > +/*
> > + * map_data->force_pte is true in order to force creating PAGE_SIZE PTEs.
> > + * data->addr is 0 because the IPA is irrelevant for a removed table.
> > + */
> > +int kvm_pgtable_stage2_create_removed(struct kvm_pgtable *pgt,
> > +                                     kvm_pte_t *new, u64 phys, u32 level,
> > +                                     enum kvm_pgtable_prot prot, void *mc)
> > +{
> > +       struct stage2_map_data map_data = {
> > +               .phys           = phys,
> > +               .mmu            = pgt->mmu,
> > +               .memcache       = mc,
> > +               .force_pte      = true,
> > +       };
> > +       struct kvm_pgtable_walker walker = {
> > +               .cb             = stage2_map_walker,
> > +               .flags          = KVM_PGTABLE_WALK_LEAF |
> > +                                 KVM_PGTABLE_WALK_REMOVED,
> > +               .arg            = &map_data,
> > +       };
> > +       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, pgtable, level + 1);
> > +       if (ret) {
> > +               kvm_pgtable_stage2_free_removed(mm_ops, pgtable, level);
> > +               mm_ops->put_page(pgtable);
> > +               return ret;
> > +       }
> > +
> > +       *new = kvm_init_table_pte(pgtable, mm_ops);
> > +       return 0;
> > +}
> >
> >  int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
> >                               struct kvm_pgtable_mm_ops *mm_ops,
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >