On Wed, May 21, 2025, Sean Christopherson wrote:
> On Wed, May 21, 2025, Yan Zhao wrote:
> > On Fri, May 16, 2025 at 02:35:39PM -0700, Sean Christopherson wrote:
> > > @@ -141,42 +140,42 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
> > > ring->reset_index++;
> > > (*nr_entries_reset)++;
> > >
> > > - /*
> > > - * While the size of each ring is fixed, it's possible for the
> > > - * ring to be constantly re-dirtied/harvested while the reset
> > > - * is in-progress (the hard limit exists only to guard against
> > > - * wrapping the count into negative space).
> > > - */
> > > - if (!first_round)
> > > + if (mask) {
> > > + /*
> > > + * While the size of each ring is fixed, it's possible
> > > + * for the ring to be constantly re-dirtied/harvested
> > > + * while the reset is in-progress (the hard limit exists
> > > + * only to guard against the count becoming negative).
> > > + */
> > > cond_resched();
> > >
> > > - /*
> > > - * Try to coalesce the reset operations when the guest is
> > > - * scanning pages in the same slot.
> > > - */
> > > - if (!first_round && next_slot == cur_slot) {
> > > - s64 delta = next_offset - cur_offset;
> > > + /*
> > > + * Try to coalesce the reset operations when the guest
> > > + * is scanning pages in the same slot.
> > > + */
> > > + if (next_slot == cur_slot) {
> > > + s64 delta = next_offset - cur_offset;
> > >
> > > - if (delta >= 0 && delta < BITS_PER_LONG) {
> > > - mask |= 1ull << delta;
> > > - continue;
> > > - }
> > > + if (delta >= 0 && delta < BITS_PER_LONG) {
> > > + mask |= 1ull << delta;
> > > + continue;
> > > + }
> > >
> > > - /* Backwards visit, careful about overflows! */
> > > - if (delta > -BITS_PER_LONG && delta < 0 &&
> > > - (mask << -delta >> -delta) == mask) {
> > > - cur_offset = next_offset;
> > > - mask = (mask << -delta) | 1;
> > > - continue;
> > > + /* Backwards visit, careful about overflows! */
> > > + if (delta > -BITS_PER_LONG && delta < 0 &&
> > > + (mask << -delta >> -delta) == mask) {
> > > + cur_offset = next_offset;
> > > + mask = (mask << -delta) | 1;
> > > + continue;
> > > + }
> > > }
> > > - }
> > >
> > > - /*
> > > - * Reset the slot for all the harvested entries that have been
> > > - * gathered, but not yet fully processed.
> > > - */
> > > - if (mask)
> > > + /*
> > > + * Reset the slot for all the harvested entries that
> > > + * have been gathered, but not yet fully processed.
> > > + */
> > > kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> > Nit and feel free to ignore it :)
> >
> > Would it be better to move the "cond_resched()" to here, i.e., executing it for
> > at most every 64 entries?
>
> Hmm, yeah, I think that makes sense. The time spent manipulating the ring and
> mask+offset is quite trivial, so checking on every single entry is unnecessary.
Oh, no, scratch that. Thankfully, past me explicitly documented this. From
patch 3:
Note! Take care to check for reschedule even in the "continue" paths,
as a pathological scenario (or malicious userspace) could dirty the same
gfn over and over, i.e. always hit the continue path.
A batch isn't guaranteed to be flushed after processing 64 entries, it's only
flushed when an entry more than N gfns away is encountered.
