Re: [Qemu-devel] [qemu-s390x] [PATCH for-4.2 v5 1/2] kvm: s390: split too big memory section on several memslots

[Christian Borntraeger]

On 29.08.19 14:31, Igor Mammedov wrote:
> On Thu, 29 Aug 2019 14:07:44 +0200
> Christian Borntraeger <address@hidden> wrote:
> 
>> On 29.08.19 14:04, Igor Mammedov wrote:
>>> On Thu, 29 Aug 2019 08:47:49 +0200
>>> Christian Borntraeger <address@hidden> wrote:
>>>   
>>>> On 27.08.19 14:56, Igor Mammedov wrote:  
>>>>> On Tue, 20 Aug 2019 18:07:27 +0200
>>>>> Cornelia Huck <address@hidden> wrote:
>>>>>     
>>>>>> On Wed,  7 Aug 2019 11:32:41 -0400
>>>>>> Igor Mammedov <address@hidden> wrote:
>>>>>>    
>>>>>>> Max memslot size supported by kvm on s390 is 8Tb,
>>>>>>> move logic of splitting RAM in chunks upto 8T to KVM code.
>>>>>>>
>>>>>>> This way it will hide KVM specific restrictions in KVM code
>>>>>>> and won't affect baord level design decisions. Which would allow
>>>>>>> us to avoid misusing memory_region_allocate_system_memory() API
>>>>>>> and eventually use a single hostmem backend for guest RAM.
>>>>>>>
>>>>>>> Signed-off-by: Igor Mammedov <address@hidden>
>>>>>>> ---
>>>>>>> v5:
>>>>>>>   * move computation 'size -= slot_size' inside of loop body
>>>>>>>           (David Hildenbrand <address@hidden>)
>>>>>>> v4:
>>>>>>>   * fix compilation issue
>>>>>>>           (Christian Borntraeger <address@hidden>)
>>>>>>>   * advance HVA along with GPA in kvm_set_phys_mem()
>>>>>>>           (Christian Borntraeger <address@hidden>)
>>>>>>>
>>>>>>> patch prepares only KVM side for switching to single RAM memory region
>>>>>>> another patch will take care of  dropping manual RAM partitioning in
>>>>>>> s390 code.      
>>>>>>
>>>>>> I may have lost track a bit -- what is the status of this patch (and
>>>>>> the series)?    
>>>>>
>>>>> Christian,
>>>>>
>>>>> could you test it on a host that have sufficient amount of RAM?    
>>>>
>>>>
>>>> This version looks good. I was able to start a 9TB guest.
>>>> [pid 215723] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0, 
>>>> guest_phys_addr=0, memory_size=8796091973632, 
>>>> userspace_addr=0x3ffee700000}) = 0
>>>> [pid 215723] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=1, flags=0, 
>>>> guest_phys_addr=0x7fffff00000, memory_size=1099512676352, 
>>>> userspace_addr=0xbffee600000}) = 0
>>
>>>> The only question is if we want to fix the weird alignment (0x7fffff00000) 
>>>> when
>>>> we already add a migration barrier for uber-large guests.
>>>> Maybe we could split at 4TB to avoid future problem with larger page 
>>>> sizes?  
>>> That probably should be a separate patch on top.  
>>
>> Right. The split in KVM code is transparent to migration and other parts of 
>> QEMU, correct?
> 
> it should not affect other QEMU parts and migration (to my limited 
> understanding of it),
> we are passing to KVM memory slots upto KVM_SLOT_MAX_BYTES as we were doing 
> before by
> creating several memory regions instead of one as described in [2/2] commit 
> message.
> 
> Also could you also test migration of +9Tb guest, to check that nothing where 
> broken by
> accident in QEMU migration code?

I only have one server that is large enough :-/