On Mon, Jan 27, 2025 at 11:57 AM James Houghton <jthoughton@xxxxxxxxxx> wrote:
>
> On Tue, Nov 5, 2024 at 10:43 AM James Houghton <jthoughton@xxxxxxxxxx> wrote:
> >
> > static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 4508d868f1cd..f5b4f1060fff 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -178,6 +178,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> > ((_only_valid) && (_root)->role.invalid))) { \
> > } else
> >
> > +/*
> > + * Iterate over all TDP MMU roots in an RCU read-side critical section.
> > + */
> > +#define for_each_valid_tdp_mmu_root_rcu(_kvm, _root, _as_id) \
> > + list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link) \
> > + if ((_as_id >= 0 && kvm_mmu_page_as_id(_root) != _as_id) || \
> > + (_root)->role.invalid) { \
> > + } else
> > +
>
> Venkatesh noticed that this function is unused in this patch. This was
> a mistake in the latest rebase. The diff should have been applied:
>
> @@ -1192,15 +1206,15 @@ static bool __kvm_tdp_mmu_age_gfn_range(struct kvm *kvm,
> struct tdp_iter iter;
> bool ret = false;
>
> + guard(rcu)();
> +
> /*
> * Don't support rescheduling, none of the MMU notifiers that funnel
> * into this helper allow blocking; it'd be dead, wasteful code. Note,
> * this helper must NOT be used to unmap GFNs, as it processes only
> * valid roots!
> */
> - for_each_valid_tdp_mmu_root(kvm, root, range->slot->as_id) {
> - guard(rcu)();
> -
> + for_each_valid_tdp_mmu_root_rcu(kvm, root, range->slot->as_id) {
> tdp_root_for_each_leaf_pte(iter, root, range->start,
> range->end) {
> if (!is_accessed_spte(iter.old_spte))
> continue;
>
> This bug will show up as a LOCKDEP warning on CONFIG_PROVE_RCU_LIST=y
> kernels, as list_for_each_entry_rcu() is called outside of an RCU
> read-side critical section.
There I go lying again. The LOCKDEP warning that will show up is the
lockdep_assert_held_write(mmu_lock) in
kvm_lockdep_assert_mmu_lock_held().
The RCU lockdep WARN would hit if I had used
for_each_valid_tdp_mmu_root_rcu() without moving the RCU lock
acquisition.
