On 12/08/21 19:46, Sean Christopherson wrote:
if (iter->level == iter->min_level)
return false;
/*
* Reread the SPTE before stepping down to avoid traversing into page
* tables that are no longer linked from this entry.
*/
iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep)); \
---> this is the code that is avoided
child_pt = spte_to_child_pt(iter->old_spte, iter->level); /
if (!child_pt)
return false;
Ah, right - so I agree with Ben that it's not too important.
Ya. There is a measurable performance improvement, but it's really only
meaningful when there aren't many SPTEs to zap, otherwise the cost of zapping
completely dominates the time.
I don't understand. When try_step_down is called by tdp_iter_next, all
it does is really just the READ_ONCE, because spte_to_child_pt will see
a non-present PTE and return immediately. Why do two, presumably cache
hot, reads cause a measurable performance improvement?
Paolo
