On Fri, Aug 13, 2021, Paolo Bonzini wrote:
On 12/08/21 19:46, Sean Christopherson wrote:
Ya. There is a measurable performance improvement, but it's really onlyif (iter->level == iter->min_level)Ah, right - so I agree with Ben that it's not too important.
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;
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?
It's entirely possible my measurements were bad and/or noisy. Ah, and my kernel
was running with CONFIG_PROVE_RCU=y, which makes the rcu_dereference() quite a bit
more expensive.