Re: [PATCH v7 0/4] s390: stop abusing memory_region_allocate_system_memory()

[Christian Borntraeger]

On 24.09.19 16:47, Igor Mammedov wrote:
> Changelog:
>   since v6:
>     - include and rebase on top of
>        [PATCH 0/2] kvm: clear dirty bitmaps from all overlapping memslots
>         https://www.mail-archive.com/address@hidden/msg646200.html
>     - minor fixups suggested during v6 review
>     - more testing incl. hacked x86
>   since v5:
>     - [1/2] fix migration that wasn't starting and make sure that KVM part
>       is able to handle 1:n MemorySection:memslot arrangement
>   since v3:
>     - fix compilation issue
>     - advance HVA along with GPA in kvm_set_phys_mem()
>   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, the later was modified to deal with 
> memory section split on several KVMSlots and manual RAM splitting in s390
> was replace by single memory_region_allocate_system_memory() call.
> 
> Tested:
>   * s390 with hacked KVM_SLOT_MAX_BYTES = 128Mb
>       - guest reboot cycle in ping-pong migration
>   * x86 with hacke max memslot = 128 and manual_dirty_log_protect enabled
>       - ping-pong migration with workload dirtying RAM around a split area
> 

Thanks, v7 applied to s390-next.