On Tue, Jul 27, 2021 at 03:21:31PM +0200, David Hildenbrand wrote:
> On 23.07.21 21:34, Peter Xu wrote:
> > Topology update could be wrongly triggered in memory region finalize() if
> > there's bug somewhere else. It'll be a very confusing stack when it
> > happens (e.g., sending KVM ioctl within the RCU thread, and we'll observe it
> > only until it fails!).
> >
> > Instead of that, we use the push()/pop() helper to avoid memory transaction
> > commit, at the same time we use assertions to make sure there's no pending
> > updates or it's a nested transaction, so it could fail even earlier and in a
> > more explicit way.
> >
> > Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
> > Signed-off-by: Peter Xu <pet...@redhat.com>
> > ---
> > softmmu/memory.c | 23 +++++++++++++++++++++--
> > 1 file changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/softmmu/memory.c b/softmmu/memory.c
> > index 1a3e9ff8ad..dfce4a2bda 100644
> > --- a/softmmu/memory.c
> > +++ b/softmmu/memory.c
> > @@ -170,6 +170,12 @@ struct MemoryRegionIoeventfd {
> > EventNotifier *e;
> > };
> > +/* Returns whether there's any pending memory updates */
> > +static bool memory_region_has_pending_update(void)
> > +{
> > + return memory_region_update_pending || ioeventfd_update_pending;
> > +}
> > +
> > static bool memory_region_ioeventfd_before(MemoryRegionIoeventfd *a,
> > MemoryRegionIoeventfd *b)
> > {
> > @@ -1756,12 +1762,25 @@ static void memory_region_finalize(Object *obj)
> > * and cause an infinite loop.
> > */
> > mr->enabled = false;
> > - memory_region_transaction_begin();
> > +
> > + /*
> > + * Use push()/pop() instead of begin()/commit() to make sure below
> > block
> > + * won't trigger any topology update (which should never happen, but
> > it's
> > + * still a safety belt).
> > + */
>
> Hmm, I wonder if we can just keep the begin/end semantics and just do an
> assertion before doing the commit? Does anything speak against that?
That sounds working too for the case of run_on_cpu and similar, but I think
this patch should be able to cover more. For example, it's possible depth==0
when enter memory_region_finalize(), but some removal of subregions could
further cause memory layout changes. IMHO we should also bail out early for
those cases too. Thanks,
--
Peter Xu
