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

[Peter Xu <peterx@redhat.com>]

On Fri, Sep 27, 2019 at 03:33:20PM +0200, Igor Mammedov wrote:
> On Thu, 26 Sep 2019 07:52:35 +0800
> Peter Xu <peterx@redhat.com> wrote:
> 
> > 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,
> Another question, Is there another archs with that much RAM that are
> available/used in real life (if not I'd wait for demand to arise first)?

I don't know, so it was a pure question besides the series.  Sorry if
that holds your series somehow, it was not my intention.

> 
> If we are to generalize it to other targets, then instead of using
> arbitrary memslot max size per target, we could just hardcode or get
> from KVM, max supported size of bitmap and use that to calculate
> kvm_max_slot_size depending on target page size.

Right, I think if so hard code would be fine for now, and probably can
with a smallest one across all archs (should depend on the smallest
page size, I guess).

> 
> Then there wouldn't be need for having machine specific code
> to care about it and pick/set arbitrary values.
> 
> Another aspect to think about if we are to enable it for
> other targets is memslot accounting. It doesn't affect s390
> but other targets that support memory hotplug now assume 1:1
> relation between memoryregion:memslot, which currently holds
> true but would need to amended in case split is enabled there.

I didn't know this.  So maybe it makes more sense to have s390 only
here.  Thanks,

-- 
Peter Xu