Re: [qemu-s390x] [PATCH/RFC] s390x: split memory into 4TB chunks

[Christian Borntraeger]

On 12/06/2017 01:19 PM, David Hildenbrand wrote:
> 
>>>>> This will most certainly break migration, no?
>>>>
>>>> Probably yes. Thats why the patch has RFC. I was looking for ideas.
>>>
>>> As other architectures might also run into this problem, wonder if
>>>
>>> a) bumping up KVM_MEM_MAX_NR_PAGES makes sense.
>>
>> The original limitation comes from the fact that this define is used to limit
>> the number of bits in the dirty bitmap as some architectures do not provide
>> bitops beyond 2^32.
> 
> Then my question would be if we can bump it up for all architectures
> that support it.
> 
>>
>>> b) as I said, transparently handle it in kvm slot handling code.
>>
>> adding Paolo to check what he thinks.

Another quick variant would be to keep the name of the first memslot as 
is (to be compatible) and start a new one just after the maximum size of.
After all migration will fail anyway for larger guests.

>>>
>>>>>
>>>>> 1. I remember the name being used for migration, I might be wrong.
>>>>> 2. Migration of guests > 4TB is certainly broken ;)
>>>>>
>>>>> I wonder if this should rather be handled in the kvm_slot code.
>>>>> (silently create an manage two slots)
>>>>
>>>
>>>
>>
> 
>