On Tue, Nov 15, 2022 at 03:03:42PM -0800, Ricardo Koller wrote:
> On Mon, Nov 14, 2022 at 08:54:52PM +0000, Oliver Upton wrote:
> > Hi Ricardo,
> >
> > On Sat, Nov 12, 2022 at 08:17:06AM +0000, Ricardo Koller wrote:
> >
> > [...]
> >
> > > +/**
> > > + * 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.
> > > + *
> > > + * @addr and the end (@addr + @size) are effectively aligned down and up to
> > > + * the top level huge-page block size. This is an exampe 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--]
> > > + * : :
> > > + * [ ][ ][:][ ][ ][ ][ ][ ][:][ ][ ][ ]
> > > + * : :
> > > + *
> > > + * 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 the size of the memcache. It
> > > + * will fail it wasn't able to break any block.
> > > + */
> > > +int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size, void *mc);
> > > +
> > > /**
> > > * 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 d1f309128118..9c42eff6d42e 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -1267,6 +1267,80 @@ static int stage2_create_removed(kvm_pte_t *ptep, u64 phys, u32 level,
> > > return __kvm_pgtable_visit(&data, mm_ops, ptep, level);
> > > }
> > >
> > > +struct stage2_split_data {
> > > + struct kvm_s2_mmu *mmu;
> > > + void *memcache;
> > > + struct kvm_pgtable_mm_ops *mm_ops;
> >
> > You can also get at mm_ops through kvm_pgtable_visit_ctx
> >
> > > +};
> > > +
> > > +static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx,
> > > + enum kvm_pgtable_walk_flags visit)
> > > +{
> > > + struct stage2_split_data *data = ctx->arg;
> > > + struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
> > > + kvm_pte_t pte = ctx->old, attr, new;
> > > + enum kvm_pgtable_prot prot;
> > > + void *mc = data->memcache;
> > > + u32 level = ctx->level;
> > > + u64 phys;
> > > +
> > > + if (WARN_ON_ONCE(kvm_pgtable_walk_shared(ctx)))
> > > + return -EINVAL;
> > > +
> > > + /* Nothing to split at the last level */
> > > + if (level == KVM_PGTABLE_MAX_LEVELS - 1)
> > > + return 0;
> > > +
> > > + /* We only split valid block mappings */
> > > + if (!kvm_pte_valid(pte) || kvm_pte_table(pte, ctx->level))
> > > + return 0;
> > > +
> > > + phys = kvm_pte_to_phys(pte);
> > > + prot = kvm_pgtable_stage2_pte_prot(pte);
> > > + stage2_set_prot_attr(data->mmu->pgt, prot, &attr);
> > > +
> > > + /*
> > > + * Eager page splitting is best-effort, so we can ignore the error.
> > > + * The returned PTE (new) will be valid even if this call returns
> > > + * error: new will be a single (big) block PTE. The only issue is
> > > + * that it will affect dirty logging performance, as the huge-pages
> > > + * will have to be split on fault, and so we WARN.
> > > + */
> > > + WARN_ON(stage2_create_removed(&new, phys, level, attr, mc, mm_ops));
> >
> > I don't believe we should warn in this case, at least not
> > unconditionally. ENOMEM is an expected outcome, for example.
>
> Given that "eager page splitting" is best-effort, the error must be
> ignored somewhere: either here or by the caller (in mmu.c). It seems
> that ignoring the error here is not a very good idea.
Actually, ignoring the error here simplifies the error handling.
stage2_create_removed() is best-effort; here's an example. If
stage2_create_removed() was called to split a 1G block PTE, and it
wasn't able to split all 2MB blocks, it would return ENOMEM and a valid
PTE pointing to a tree like this:
[---------1GB-------------]
: :
[--2MB--][--2MB--][--2MB--]
: :
[ ][ ][ ]
If we returned ENOMEM instead of ignoring the error, we would have to
clean all the intermediate state. But stage2_create_removed() is
designed to always return a valid PTE, even if the tree is not fully
split (as above). So, there's no really need to clean it: it's a valid
tree. Moreover, this valid tree would result in better dirty logging
performance as it already has some 2M blocks split into 4K pages.
>
> >
> > Additionally, I believe you'll want to bail out at this point to avoid
> > installing a potentially garbage PTE as well.
>
> It should be fine as stage2_create_removed() is also best-effort. The
> returned PTE is valid even when it fails; it just returns a big block
> PTE.
>
> >
> > > + stage2_put_pte(ctx, data->mmu, mm_ops);
> >
> > Ah, I see why you've relaxed the WARN in patch 1 now.
> >
> > I would recommend you follow the break-before-make pattern and use the
> > helpers here as well. stage2_try_break_pte() will demote the store to
> > WRITE_ONCE() if called from a non-shared context.
> >
>
> ACK, I can do that. The only reason why I didnt' is because I would have
> to handle the potential error from stage2_try_break_pte(). It would feel
> wrong not to, even if it's !shared. On the other hand, I would like to
> easily experiment with both the !shared and the shared approaches
> easily.
>
> > Then the WARN will behave as expected in stage2_make_pte().
> >
> > > + /*
> > > + * 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().
> >
> > typo: stage2_make_pte()
> >
> > --
> > Thanks,
> > Oliver
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