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

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 25, 2019 at 01:51:05PM +0200, Igor Mammedov wrote:
> 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

I'm sorry, I overlooked that.

>  
> > 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?

Yes I thought we had.  So I feel like it would be good to also allow
other archs to support >8TB mem as well.  Thanks,

-- 
Peter Xu