Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

Peter Xu
2019-12-16T15:08:01Z

On Mon, Dec 16, 2019 at 04:47:36AM -0500, Michael S. Tsirkin wrote:
> On Sun, Dec 15, 2019 at 12:33:02PM -0500, Peter Xu wrote:
> > On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote:
> > > >>> What depends on what here? Looks suspicious ...
> > > >>
> > > >> Hmm, I think maybe it can be removed because the entry pointer
> > > >> reference below should be an ordering constraint already?
> > >
> > > entry->xxx depends on ring->reset_index.
> >
> > Yes that's true, but...
> >
> > entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
> > /* barrier? */
> > next_slot = READ_ONCE(entry->slot);
> > next_offset = READ_ONCE(entry->offset);
> >
> > ... I think entry->xxx depends on entry first, then entry depends on
> > reset_index. So it seems fine because all things have a dependency?
>
> Is reset_index changed from another thread then?
> If yes then you want to read reset_index with READ_ONCE.
> That includes a dependency barrier.

There're a few readers, but only this function will change it
(kvm_dirty_ring_reset). Thanks,

--
Peter Xu