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

[David Hildenbrand <david@redhat.com>]

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.

-- 

Thanks,

David / dhildenb