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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 25 Sep 2019 11:27:00 +0800
Peter Xu <peterx@redhat.com> wrote:

> 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.

/me confused, bb223055b9 is mentioned in commit message
 
> Also, this only applies the split limitation to s390.  Would that be a
> good thing to some other archs as well?

Don't we have the similar bitmap size issue in KVM for other archs?

> 
> Thanks,
>