Re: [qemu-s390x] [PATCH RFC 0/2] s390: stop abusing memory_region_allocate_system_memory()

[David Hildenbrand]

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.

Do we *really* care about keeping migration of systems running that most
probably nobody (except Christian ;) ) really uses? (especially not in
production).

I am fine keeping migration running if it's easy, but introducing hacks
(reading below) for such obscure use cases - I don't know.

@Christian: Please prove me wrong. :)

> 
> In order to replace these several RAM chunks with a single memdev and keep it
> working with KVM memslot size limit and migration compatible, following was 
> done:
>    * [2/2] use memory region aliases to partition hostmem backend RAM on
>            KVM_SLOT_MAX_BYTES chunks, which should keep KVM side working
>    * [1/2] hacked memory region aliases (to ram memory regions only) to have
>            its own RAMBlocks pointing to RAM chunks owned by aliased memory
>            region. While it's admittedly a hack, but it's relatively simple 
> and
>            allows board code rashape migration stream as necessary
> 
>            I haven't tried to use migratable aliases on x86 machines, but 
> with it
>            it could be possible to drop legacy RAM allocation and compat knob
>            (cd5ff8333a) dropping '-numa node,mem' completely even for old 
> machines.
> 
> PS:
>    Tested with ping pong cross version migration on s390 machine 
>    (with reduced KVM_SLOT_MAX_BYTES since I don't have access to large
>     enough host)
>      
> 
> Igor Mammedov (2):
>   memory: make MemoryRegion alias migratable
>   s390: do not call memory_region_allocate_system_memory() multiple
>     times
> 
>  exec.c                     |  7 ++++---
>  hw/s390x/s390-virtio-ccw.c | 20 +++++++++++++++-----
>  memory.c                   |  5 +++++
>  3 files changed, 24 insertions(+), 8 deletions(-)
> 


-- 

Thanks,

David / dhildenb