Re: [PATCH v7 4/4] s390: do not call memory_region_allocate_system_memory() multiple times

[Christian Borntraeger]

On 30.09.19 11:33, Igor Mammedov wrote:
> On Mon, 30 Sep 2019 09:09:59 +0200
> Christian Borntraeger <address@hidden> wrote:
> 
>> On 28.09.19 03:28, Peter Xu wrote:
>>> On Fri, Sep 27, 2019 at 03:33:20PM +0200, Igor Mammedov wrote:  
>>>> On Thu, 26 Sep 2019 07:52:35 +0800
>>>> Peter Xu <address@hidden> wrote:
>>>>  
>>>>> On Wed, Sep 25, 2019 at 01:51:05PM +0200, Igor Mammedov wrote:  
>>>>>> On Wed, 25 Sep 2019 11:27:00 +0800
>>>>>> Peter Xu <address@hidden> wrote:
>>>>>>     
>>>>>>> On Tue, Sep 24, 2019 at 10:47:51AM -0400, Igor Mammedov wrote:    
>>>>>>>> s390 was trying to solve limited KVM memslot size issue by abusing
>>>>>>>> memory_region_allocate_system_memory(), which breaks API contract
>>>>>>>> where the function might be called only once.
>>>>>>>>
>>>>>>>> Beside an invalid use of API, the approach also introduced migration
>>>>>>>> issue, since RAM chunks for each KVM_SLOT_MAX_BYTES are transferred in
>>>>>>>> migration stream as separate RAMBlocks.
>>>>>>>>
>>>>>>>> After discussion [1], it was agreed to break migration from older
>>>>>>>> QEMU for guest with RAM >8Tb (as it was relatively new (since 2.12)
>>>>>>>> and considered to be not actually used downstream).
>>>>>>>> Migration should keep working for guests with less than 8TB and for
>>>>>>>> more than 8TB with QEMU 4.2 and newer binary.
>>>>>>>> In case user tries to migrate more than 8TB guest, between incompatible
>>>>>>>> QEMU versions, migration should fail gracefully due to non-exiting
>>>>>>>> RAMBlock ID or RAMBlock size mismatch.
>>>>>>>>
>>>>>>>> Taking in account above and that now KVM code is able to split too
>>>>>>>> big MemorySection into several memslots, partially revert commit
>>>>>>>>  (bb223055b s390-ccw-virtio: allow for systems larger that 7.999TB)
>>>>>>>> and use kvm_set_max_memslot_size() to set KVMSlot size to
>>>>>>>> KVM_SLOT_MAX_BYTES.
>>>>>>>>
>>>>>>>> 1) [PATCH RFC v2 4/4] s390: do not call  
>>>>>>>> memory_region_allocate_system_memory() multiple times
>>>>>>>>
>>>>>>>> Signed-off-by: Igor Mammedov <address@hidden>      
>>>>>>>
>>>>>>> Acked-by: Peter Xu <address@hidden>
>>>>>>>
>>>>>>> IMHO it would be good to at least mention bb223055b9 in the commit
>>>>>>> message even if not with a "Fixed:" tag.  May be amended during commit
>>>>>>> if anyone prefers.    
>>>>>>
>>>>>> /me confused, bb223055b9 is mentioned in commit message    
>>>>>
>>>>> I'm sorry, I overlooked that.
>>>>>  
>>>>>>      
>>>>>>> Also, this only applies the split limitation to s390.  Would that be a
>>>>>>> good thing to some other archs as well?    
>>>>>>
>>>>>> Don't we have the similar bitmap size issue in KVM for other archs?    
>>>>>
>>>>> Yes I thought we had.  So I feel like it would be good to also allow
>>>>> other archs to support >8TB mem as well.  Thanks,  
>>>> Another question, Is there another archs with that much RAM that are
>>>> available/used in real life (if not I'd wait for demand to arise first)?  
>>>
>>> I don't know, so it was a pure question besides the series.  Sorry if
>>> that holds your series somehow, it was not my intention.
>>>   
>>>>
>>>> If we are to generalize it to other targets, then instead of using
>>>> arbitrary memslot max size per target, we could just hardcode or get
>>>> from KVM, max supported size of bitmap and use that to calculate
>>>> kvm_max_slot_size depending on target page size.  
>>>
>>> Right, I think if so hard code would be fine for now, and probably can
>>> with a smallest one across all archs (should depend on the smallest
>>> page size, I guess).
>>>   
>>>>
>>>> Then there wouldn't be need for having machine specific code
>>>> to care about it and pick/set arbitrary values.
>>>>
>>>> Another aspect to think about if we are to enable it for
>>>> other targets is memslot accounting. It doesn't affect s390
>>>> but other targets that support memory hotplug now assume 1:1
>>>> relation between memoryregion:memslot, which currently holds
>>>> true but would need to amended in case split is enabled there.  
>>>
>>> I didn't know this.  So maybe it makes more sense to have s390 only
>>> here.  Thanks,  
>>
>> OK. So shall I take the series as is via the s390 tree?
> Yes, I'd appreciate it.


Paolo, ok it I pick the first 3 patches as well? Can you ack?