Re: [Qemu-devel] [RFC v2 3/6] memory: support unmapping of MemoryRegion mapped into HVA parent

[Igor Mammedov]

On Wed, 17 Jun 2015 10:14:00 +0200
Paolo Bonzini <address@hidden> wrote:

> 
> 
> On 09/06/2015 12:08, Igor Mammedov wrote:
> > 
> > 3. until #2 is done we can't allow to map another memory region in
> >    current flatview at the same range, hence need to keep list of
> >    still active HVA ranges so we could check at 
> > memory_region_add_subregion()
> >    time that new mapping is allowed. => adding "Error **errp"
> >    to memory_region_add_subregion() for reporting fail.
> 
> No, we cannot do that.  memory_region_add_subregion cannot fail.  The
> RCU mechanism is transparent.
So what are the options?

Actually memory_region_add_subregion(cannot fail) will continue to casue 
problems
in case one of the listeners fails and has no way to propagate error up the 
stack.
It could be that vhost_set_memory() or kvm_set_memslots() aren't able to 
allocate
memory for replacement table in low memory conditions and without
way to report back QEMU is left with option to die.

> 
> Paolo