Re: [Qemu-devel] [PATCH for-4.2 v3 0/2] s390: stop abusing memory_region_allocate_system_memory()

[Christian Borntraeger]

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. 
> 
> Igor Mammedov (2):
>   kvm: s390: split too big memory section on several memslots
>   s390: do not call memory_region_allocate_system_memory() multiple
>     times
> 
>  include/hw/s390x/s390-virtio-ccw.h | 10 ++++
>  include/sysemu/kvm_int.h           |  1 +
>  accel/kvm/kvm-all.c                | 79 ++++++++++++++++++------------
>  hw/s390x/s390-virtio-ccw.c         | 30 ++----------
>  target/s390x/kvm.c                 |  1 +
>  5 files changed, 63 insertions(+), 58 deletions(-)
>