Re: [qemu-s390x] [PATCH RFCv2] s390x/sclp: remove memory hotplug support

[David Hildenbrand]

On 20.02.2018 15:57, Cornelia Huck wrote:
> On Tue, 20 Feb 2018 13:16:37 +0100
> David Hildenbrand <address@hidden> wrote:
> 
>> On 20.02.2018 13:05, Christian Borntraeger wrote:
>>>
>>>
>>> On 02/19/2018 06:42 PM, David Hildenbrand wrote:  
>>>> From an architecture point of view, nothing can be mapped into the address
>>>> space on s390x. All there is is memory. Therefore there is also not really
>>>> an interface to communicate such information to the guest. All we can do is
>>>> specify the maximum ram address and guests can probe in that range if
>>>> memory is available and usable (TPROT).  
>>>
>>> In fact there is an interface in SCLP that describes the memory sizes 
>>> (maximum in 
>>> read scp info) and the details (read_storage_element0_info).  I am asking 
>>> myself
>>> if we should re-introduce read_storage_element_info and use that to avoid 
>>> tprot  
>>
>> Yes, we could do that (basically V1 of this patch) but have to glue it
>> to the a compatibility machine then.
> 
> Actually, this makes quite a bit of sense (introduce the interface for
> everyone in 2.12 and turn it off in compat machines).

Jup, either 2.12 or 2.13, no need to hurry.

> 
> Does real hardware have configurations where you can get the memory
> sizes, but not the attach/deattach support? (Hardware with the feature,
> but no standby memory defined?)

I would guess that "0" for standby memory is valid but only people with
access to documentation can answer that :)

>> Interesting, didn't know about that. Will rephrase then to
>>
>> "While the hypervisor can deny to online an increment, all increments
>> have to be predefined and there is no way of telling the guest about a
>> newly "hotplugged" increment."
> 
> Rephrase which part? :)

"And nobody can really hinder it from doing so."

Thanks!

-- 

Thanks,

David / dhildenb