Re: [PATCH v7 4/4] s390: do not call memory_region_allocate_system_memory() multiple times

[Peter Xu <peterx@redhat.com>]

On Tue, Sep 24, 2019 at 10:47:51AM -0400, Igor Mammedov wrote:
> s390 was trying to solve limited KVM memslot size issue by abusing
> memory_region_allocate_system_memory(), which breaks API contract
> where the function might be called only once.
> 
> Beside an invalid use of API, the approach also introduced migration
> issue, since RAM chunks for each KVM_SLOT_MAX_BYTES are transferred in
> migration stream as separate RAMBlocks.
> 
> After discussion [1], it was agreed to break migration from older
> QEMU for guest with RAM >8Tb (as it was relatively new (since 2.12)
> and considered to be not actually used downstream).
> Migration should keep working for guests with less than 8TB and for
> more than 8TB with QEMU 4.2 and newer binary.
> In case user tries to migrate more than 8TB guest, between incompatible
> QEMU versions, migration should fail gracefully due to non-exiting
> RAMBlock ID or RAMBlock size mismatch.
> 
> Taking in account above and that now KVM code is able to split too
> big MemorySection into several memslots, partially revert commit
>  (bb223055b s390-ccw-virtio: allow for systems larger that 7.999TB)
> and use kvm_set_max_memslot_size() to set KVMSlot size to
> KVM_SLOT_MAX_BYTES.
> 
> 1) [PATCH RFC v2 4/4] s390: do not call  memory_region_allocate_system_memory() multiple times
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>

Acked-by: Peter Xu <peterx@redhat.com>

IMHO it would be good to at least mention bb223055b9 in the commit
message even if not with a "Fixed:" tag.  May be amended during commit
if anyone prefers.

Also, this only applies the split limitation to s390.  Would that be a
good thing to some other archs as well?

Thanks,

-- 
Peter Xu