On Mon, 30 Sep 2019 09:09:59 +0200 Christian Borntraeger <borntrae...@de.ibm.com> wrote:
> On 28.09.19 03:28, Peter Xu wrote: > > 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, > > OK. So shall I take the series as is via the s390 tree? Yes, I'd appreciate it. > I would like to add the following patch on top if nobody minds: > > Subject: [PATCH 1/1] s390/kvm: split kvm mem slots at 4TB > > Instead of splitting at an unaligned address, we can simply split at > 4TB. > > Signed-off-by: Christian Borntraeger <borntrae...@de.ibm.com> Looks fine to me Acked-by: Igor Mammedov <imamm...@redhat.com> > --- > target/s390x/kvm.c | 9 ++++----- > 1 file changed, 4 insertions(+), 5 deletions(-) > > diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c > index ad2dd14f7e78..611f56f4b5ac 100644 > --- a/target/s390x/kvm.c > +++ b/target/s390x/kvm.c > @@ -126,12 +126,11 @@ > /* > * KVM does only support memory slots up to KVM_MEM_MAX_NR_PAGES pages > * as the dirty bitmap must be managed by bitops that take an int as > - * position indicator. If we have a guest beyond that we will split off > - * new subregions. The split must happen on a segment boundary (1MB). > + * position indicator. This would end at an unaligned address > + * (0x7fffff00000). As future variants might provide larger pages > + * and to make all addresses properly aligned, let us split at 4TB. > */ > -#define KVM_MEM_MAX_NR_PAGES ((1ULL << 31) - 1) > -#define SEG_MSK (~0xfffffULL) > -#define KVM_SLOT_MAX_BYTES ((KVM_MEM_MAX_NR_PAGES * TARGET_PAGE_SIZE) & > SEG_MSK) > +#define KVM_SLOT_MAX_BYTES 4096UL*1024*1024*1024 I'd use TiB instead of 1024*1024*1024 > > static CPUWatchpoint hw_watchpoint; > /*
