Re: [Qemu-devel] [PATCH v1 5/5] s390: do not call memory_region_allocate_system_memory() multiple times

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 18 Apr 2019 13:24:43 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 18.04.19 11:38, Igor Mammedov wrote:
> > On Tue, 16 Apr 2019 13:09:08 +0200
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >   
> >> This fails with more than 8TB, e.g.  "-m 9T "
> >>
> >> [pid 231065] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0, guest_phys_addr=0, memory_size=0, userspace_addr=0x3ffc8500000}) = 0
> >> [pid 231065] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0, guest_phys_addr=0, memory_size=9895604649984, userspace_addr=0x3ffc8500000}) = -1 EINVAL (Invalid argument)
> >>
> >> seems that the 2nd memslot gets the full size (and not 9TB-size of first slot).  
> > 
> > it turns out MemoryRegions is rendered correctly in to 2 parts (one per alias),
> > but follow up flatview_simplify() collapses adjacent ranges back
> > into big one.  
> 
> That sounds dangerous. Imagine doing that at runtime (e.g. hotplugging a
> DIMM), the kvm memory slot would temporarily be deleted to insert the
> new, bigger one. Guest would crash. This could happen if backing memory
> of two DIMMs would by pure luck be allocated side by side in user space.
> 

not sure I fully get your concerns, but if you look at can_merge()
it ensures that ranges belong to the same MemoryRegion.

It's hard for me to say if flatview_simplify() works as designed,
MemoryRegion code is quite complicated so I'd deffer to Paolo's
opinion.