On 8/9/22 05:26, Yan Zhao wrote:
hi Sean,
I understand this smp_rmb() is intended to prevent the reading of
p->nx_huge_page_disallowed from happening before it's set to true in
kvm_tdp_mmu_map(). Is this understanding right?
If it's true, then do we also need the smp_rmb() for read of sp->gfn in
handle_removed_pt()? (or maybe for other fields in sp in other places?)
No, in that case the barrier is provided by rcu_dereference(). In fact,
I am not sure the barriers are needed in this patch either (but the
comments are :)):
- the write barrier is certainly not needed because it is implicit in
tdp_mmu_set_spte_atomic's cmpxchg64
- the read barrier _should_ also be provided by rcu_dereference(pt), but
I'm not 100% sure about that. The reasoning is that you have
(1) iter->old spte = READ_ONCE(*rcu_dereference(iter->sptep));
...
(2) tdp_ptep_t pt = spte_to_child_pt(old_spte, level);
(3) struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
...
(4) if (sp->nx_huge_page_disallowed) {
and (4) is definitely ordered after (1) thanks to the READ_ONCE hidden
within (3) and the data dependency from old_spte to sp.
Paolo
