Re: [RFC 3/8] virtio: Add API to batch set host notifiers

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 30, 2021 at 04:17:32PM +0200, Greg Kurz wrote:
> On Tue, 30 Mar 2021 14:55:42 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > On Tue, Mar 30, 2021 at 12:17:40PM +0200, Greg Kurz wrote:
> > > On Mon, 29 Mar 2021 18:10:57 +0100
> > > Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > > On Thu, Mar 25, 2021 at 04:07:30PM +0100, Greg Kurz wrote:
> > > > > @@ -315,6 +338,10 @@ static void virtio_bus_unset_and_cleanup_host_notifiers(VirtioBusState *bus,
> > > > >  
> > > > >      for (i = 0; i < nvqs; i++) {
> > > > >          virtio_bus_set_host_notifier(bus, i + n_offset, false);
> > > > > +    }
> > > > > +    /* Let address_space_update_ioeventfds() run before closing ioeventfds */
> > > > 
> > > > assert(memory_region_transaction_depth == 0)?
> > > > 
> > > 
> > > Hmm... appart from the fact that memory_region_transaction_depth is
> > > a memory internal thing that shouldn't be exposed here, it seems to
> > > me that memory_region_transaction_depth can be != 0 when, e.g. when
> > > batching is used... or I'm missing something ?
> > > 
> > > I was actually thinking of adding some asserts for that in the
> > > memory_region_*_eventfd_full() functions introduced by patch 1.
> > > 
> > >     if (!transaction) {
> > >         memory_region_transaction_begin();
> > >     }
> > >     assert(memory_region_transaction_depth != 0);
> > 
> > In that case is it safe to call virtio_bus_cleanup_host_notifier()
> > below? I thought it depends on the transaction committing first.
> > 
> 
> Yes because the transaction ends...
> 
> > > 
> > > > > +    virtio_bus_set_host_notifier_commit(bus);
> ...                here ^^
> 
> > > > > +    for (i = 0; i < nvqs; i++) {
> > > > >          virtio_bus_cleanup_host_notifier(bus, i + n_offset);
> > > > >      }
> > > > >  }

That contradicts what you said above: "it seems to me that
memory_region_transaction_depth can be != 0 when, e.g. when batching is
used".

If memory_region_transaction_depth can be != 0 when this function is
entered then memory_region_transaction_commit() will have no effect:

  void memory_region_transaction_commit(void)
  {
      AddressSpace *as;

      assert(memory_region_transaction_depth);
      assert(qemu_mutex_iothread_locked());

      --memory_region_transaction_depth;
      if (!memory_region_transaction_depth) {
          ^--- we won't take this branch!

So the code after memory_region_transaction_commit() cannot assume that
anything was actually committed.

That's why I asked about adding assert(memory_region_transaction_depth
== 0) to guarantee that our commit takes effect immediately so that it's
safe to call virtio_bus_cleanup_host_notifier().

Stefan

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