On Fri, Sep 27, 2019 at 03:33:20PM +0200, Igor Mammedov wrote: > On Thu, 26 Sep 2019 07:52:35 +0800 > Peter Xu <pet...@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 <pet...@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 <imamm...@redhat.com> > > > > > > > > Acked-by: Peter Xu <pet...@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
