Hi Oliver.,
On Tue, 11 Jul 2023 19:04:27 +0100,
Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> Hey Marc,
>
> On Tue, Jul 11, 2023 at 11:10:01AM +0100, Marc Zyngier wrote:
> > On Wed, 28 Jun 2023 00:54:05 +0100, Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> > > +static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
> > > + enum kvm_pgtable_walk_flags visit)
> > > {
> > > - kvm_pte_t pte = 0;
> > > - stage2_update_leaf_attrs(pgt, addr, 1, 0, KVM_PTE_LEAF_ATTR_LO_S2_AF,
> > > - &pte, NULL, 0);
> > > + kvm_pte_t new = ctx->old & ~KVM_PTE_LEAF_ATTR_LO_S2_AF;
> > > + struct stage2_age_data *data = ctx->arg;
> > > +
> > > + if (!kvm_pte_valid(ctx->old) || new == ctx->old)
> > > + return 0;
> > > +
> > > + data->young = true;
> > > +
> > > + if (data->mkold && !stage2_try_set_pte(ctx, new))
> > > + return -EAGAIN;
> > > +
> > > /*
> > > * "But where's the TLBI?!", you scream.
> > > * "Over in the core code", I sigh.
> > > *
> > > * See the '->clear_flush_young()' callback on the KVM mmu notifier.
> > > */
> > > - return pte;
> > > + return 0;
> > > }
> > >
> > > -bool kvm_pgtable_stage2_is_young(struct kvm_pgtable *pgt, u64 addr)
> > > +bool kvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr,
> > > + u64 size, bool mkold)
> > > {
> > > - kvm_pte_t pte = 0;
> > > - stage2_update_leaf_attrs(pgt, addr, 1, 0, 0, &pte, NULL, 0);
> > > - return pte & KVM_PTE_LEAF_ATTR_LO_S2_AF;
> > > + struct stage2_age_data data = {
> > > + .mkold = mkold,
> > > + };
> > > + struct kvm_pgtable_walker walker = {
> > > + .cb = stage2_age_walker,
> > > + .arg = &data,
> > > + .flags = KVM_PGTABLE_WALK_LEAF,
> > > + };
> > > +
> > > + WARN_ON(kvm_pgtable_walk(pgt, addr, size, &walker));
> >
> > Do we really want a WARN_ON() here? From what I can tell, it can be
> > (trivially?) triggered by the previous function returning -EAGAIN if
> > the pte update fails in the case of a shared walk.
>
> I threw the -EAGAIN in there just due to reflexes, we're holding the MMU
> write lock at this point so stage2_try_set_pte() will always succeed. A
> tad fragile, but wanted to make it trivial to change the locking around
> stage2_age_walker() in the future.
Right, the notifier takes the write lock, ensuring that there is never
a concurrent access. Maybe a small comment above the return would
help, as '-EAGAIN' is easily interpreted as "we missed the boat, but
we'll take the next one, no big deal".
>
> The reason I wanted to have a WARN here is because we're unable to
> return an error on the MMU notifier and might need some breadcrumbs to
> debug any underlying issues in the table walker. I'd really like to keep
> it in some form.
>
> I can either replace stage2_try_set_pte() with a direct WRITE_ONCE()
> (eliminating the error path) or leave it as-is. Which do you prefer?
With the above nit addressed:
Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>
M.
--
Without deviation from the norm, progress is not possible.
