Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages

Quentin Perret
2021-12-10T14:35:57Z

On Thursday 09 Dec 2021 at 10:29:24 (+0000), Will Deacon wrote:
> On Wed, Dec 01, 2021 at 05:03:57PM +0000, Quentin Perret wrote:
> > To prepare the ground for allowing hyp stage-1 mappings to be removed at
> > run-time, update the KVM page-table code to maintain a correct refcount
> > using the ->{get,put}_page() function callbacks.
> >
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> > arch/arm64/kvm/hyp/pgtable.c | 17 ++++++++++++++---
> > 1 file changed, 14 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index f8ceebe4982e..768a58835153 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -408,8 +408,10 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > return false;
> >
> > new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> > - if (hyp_pte_needs_update(old, new))
> > + if (hyp_pte_needs_update(old, new)) {
> > smp_store_release(ptep, new);
> > + data->mm_ops->get_page(ptep);
>
> In the case where we're just updating software bits for a valid pte, doesn't
> this result in us taking a spurious reference to the page?

Ahem, yes, that is the case. I ended up with the below diff to fix it,
which I intend to fold in the next version:

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 6ad4cb2d6947..e2047d3f05a2 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -383,21 +383,6 @@ enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte)
return prot;
}

-static bool hyp_pte_needs_update(kvm_pte_t old, kvm_pte_t new)
-{
- /*
- * Tolerate KVM recreating the exact same mapping, or changing software
- * bits if the existing mapping was valid.
- */
- if (old == new)
- return false;
-
- if (!kvm_pte_valid(old))
- return true;
-
- return !WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW);
-}
-
static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
kvm_pte_t *ptep, struct hyp_map_data *data)
{
@@ -407,13 +392,16 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
if (!kvm_block_mapping_supported(addr, end, phys, level))
return false;

+ data->phys += granule;
new = kvm_init_valid_leaf_pte(phys, data->attr, level);
- if (hyp_pte_needs_update(old, new)) {
- smp_store_release(ptep, new);
+ if (old == new)
+ return true;
+ else if (!kvm_pte_valid(old))
data->mm_ops->get_page(ptep);
- }
+ else if (WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW))
+ return false;

- data->phys += granule;
+ smp_store_release(ptep, new);
return true;
}