On Thu, Oct 20, 2022 at 11:32:28AM +0300, Oliver Upton wrote:
> On Wed, Oct 19, 2022 at 11:17:43PM +0000, Sean Christopherson wrote:
> > On Fri, Oct 07, 2022, Oliver Upton wrote:
[...]
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index 02c33fccb178..6b6e1ed7ee2f 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -175,13 +175,14 @@ static int kvm_pgtable_visitor_cb(struct
> > > kvm_pgtable_walk_data *data,
> > > }
> > >
> > > static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
> > > - struct kvm_pgtable_mm_ops *mm_ops, kvm_pte_t
> > > *pgtable, u32 level);
> > > + struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t
> > > pgtable, u32 level);
> > >
> > > static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> > > struct kvm_pgtable_mm_ops *mm_ops,
> > > - kvm_pte_t *ptep, u32 level)
> > > + kvm_pteref_t pteref, u32 level)
> > > {
> > > enum kvm_pgtable_walk_flags flags = data->walker->flags;
> > > + kvm_pte_t *ptep = kvm_dereference_pteref(pteref, false);
> > > struct kvm_pgtable_visit_ctx ctx = {
> > > .ptep = ptep,
> > > .old = READ_ONCE(*ptep),
> >
> > This is where you want the protection to kick in, e.g.
> >
> > typedef kvm_pte_t __rcu *kvm_ptep_t;
> >
> > static inline kvm_pte_t kvm_read_pte(kvm_ptep_t ptep)
> > {
> > return READ_ONCE(*rcu_dereference(ptep));
> > }
> >
> > .old = kvm_read_pte(ptep),
> >
> > In other words, the pointer itself isn't that's protected, it's PTE that the
> > pointer points at that's protected.
>
> Right, but practically speaking it is the boundary at which we assert
> that protection.
>
> Anyhow, I'll look at abstracting the actual memory accesses in the
> visitors without too much mess.
Took this in a slightly different direction after playing with it for a
while. Abstracting all PTE accesses adds a lot of churn to the series.
Adding in an assertion before invoking a visitor callback (i.e. when the
raw pointer is about to be used) provides a similar degree of assurance
that we are indeed RCU-safe.
--
Thanks,
Oliver
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