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Re: [qemu-s390x] [PATCH RFC 0/2] s390: stop abusing memory_region_alloca

From: David Hildenbrand
Subject: Re: [qemu-s390x] [PATCH RFC 0/2] s390: stop abusing memory_region_allocate_system_memory()
Date: Fri, 2 Aug 2019 10:26:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

On 02.08.19 10:04, David Hildenbrand wrote:
> On 29.07.19 16:52, Igor Mammedov wrote:
>> 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.
> So if I understand it correctly, we only call
> memory_region_allocate_system_memory() in case the guest initial memory
> size exceeds KVM_SLOT_MAX_BYTES - ~8TB.

(to clarify, I meant: call it more than once)



David / dhildenb

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