Re: [Qemu-devel] [qemu-s390x] [PATCH 1/1] s390x/sclp: fix maxram calculation

[Christian Borntraeger]

On 07/30/2018 05:17 PM, David Hildenbrand wrote:
> On 30.07.2018 17:00, Christian Borntraeger wrote:
>>
>>
>> On 07/30/2018 04:34 PM, David Hildenbrand wrote:
>>> On 30.07.2018 16:09, Christian Borntraeger wrote:
>>>> We clamp down ram_size to match the sclp increment size. We do
>>>> not do the same for maxram_size, which means for large guests
>>>> with some sizes (e.g. -m 50000) maxram_size differs from ram_size.
>>>> This can break other code (e.g. CMMA migration) which uses maxram_size
>>>> to calculate the number of pages and then throws some errors.
>>>
>>> So the only problem is that the buffer size between source and target
>>> differ?
>>
>> The problem is that the target tries to access a non-existing buffer when 
>> committing all cmma value, so the kernel returns with EFAULT.
>>>
> 
> Am I wrong or does CMMA migration code really not care about which parts
> of maxram are actually used (== which memory regions are actually defined)?
> 
> If so, this looks broken to me and the right fix is to use ramsize for
> now, because it simply does not support maxram.
> 
> (I assume using some -m X,maxmem=X+Y would make it fail in the same way)
> 
> (this patch still makes sense and should be done)

I am looking for the minimal fix for 2.13 and ideally even for 2.12.1.

Can we agree on this fix and do the remaining thing later?