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Re: [qemu-s390x] [PATCH for-4.2 v3 0/2] s390: stop abusing memory_region

From: Christian Borntraeger
Subject: Re: [qemu-s390x] [PATCH for-4.2 v3 0/2] s390: stop abusing memory_region_allocate_system_memory()
Date: Fri, 2 Aug 2019 17:04:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 02.08.19 16:59, Christian Borntraeger wrote:
> On 02.08.19 16:42, Christian Borntraeger wrote:
>> On 02.08.19 15:32, Igor Mammedov wrote:
>>> Changelog:
>>>   since v2:
>>>     - break migration from old QEMU (since 2.12-4.1) for guest with >8TB RAM
>>>       and drop migratable aliases patch as was agreed during v2 review
>>>     - drop 4.2 machines patch as it's not prerequisite anymore
>>>   since v1:
>>>     - include 4.2 machines patch for adding compat RAM layout on top
>>>     - 2/4 add missing in v1 patch for splitting too big MemorySection on
>>>           several memslots
>>>     - 3/4 amend code path on alias destruction to ensure that RAMBlock is
>>>           cleaned properly
>>>     - 4/4 add compat machine code to keep old layout (migration-wise) for
>>>           4.1 and older machines 
>>> While looking into unifying guest RAM allocation to use hostmem backends
>>> for initial RAM (especially when -mempath is used) and retiring
>>> memory_region_allocate_system_memory() API, leaving only single hostmem 
>>> backend,
>>> I was inspecting how currently it is used by boards and it turns out several
>>> boards abuse it by calling the function several times (despite documented 
>>> contract
>>> forbiding it).
>>> s390 is one of such boards where KVM limitation on memslot size got 
>>> propagated
>>> to board design and memory_region_allocate_system_memory() was abused to 
>>> satisfy
>>> KVM requirement for max RAM chunk where memory region alias would suffice.
>>> Unfortunately, memory_region_allocate_system_memory() usage created 
>>> migration
>>> dependency where guest RAM is transferred in migration stream as several 
>>> RAMBlocks
>>> if it's more than KVM_SLOT_MAX_BYTES. During v2 review it was agreed to 
>>> ignore
>>> migration breakage (documenting it in release notes) and leaving only KVM 
>>> fix.
>>> In order to replace these several RAM chunks with a single memdev and keep 
>>> it
>>> working with KVM memslot size limit, following was done:
>>>    * [1/2] split too big RAM chunk inside of KVM code on several memory 
>>> slots
>>>            if necessary
>>>    * [2/2] drop manual ram splitting in s390 code
>>> CC: address@hidden
>>> CC: address@hidden
>>> CC: address@hidden
>>> CC: address@hidden
>>> CC: address@hidden
>>> CC: address@hidden
>> With the fixup this patch set seems to work on s390. I can start 9TB guests 
>> and
>> I can migrate smaller guests between 4.1+patch and 4.0 and 3.1. I currently 
>> can
>> not test migration for the 9TB guest due to lack of a 2nd system. 
> I have to correct myself. The 9TB guest started up but it does not seem to do
> anything useful (it hangs).

Seems that the userspace addr is wrong (its the same). 
[pid 258234] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0, 
guest_phys_addr=0, memory_size=8796091973632, userspace_addr=0x3fff7d00000}) = 0
[pid 258234] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=1, flags=0, 
guest_phys_addr=0x7fffff00000, memory_size=1099512676352, 
userspace_addr=0x3fff7d00000}) = 0

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