On Wed, 27 Mar 2019 15:01:37 +0100 David Hildenbrand <da...@redhat.com> wrote:
> On 27.03.19 10:09, Igor Mammedov wrote: > > On Wed, 27 Mar 2019 09:10:01 +0100 > > David Hildenbrand <da...@redhat.com> wrote: > > > >> On 27.03.19 01:12, David Gibson wrote: > >>> On Tue, Mar 26, 2019 at 06:02:51PM +0100, David Hildenbrand wrote: > >>>> On 26.03.19 15:08, Igor Mammedov wrote: > >>>>> On Tue, 26 Mar 2019 14:50:58 +1100 > >>>>> David Gibson <da...@gibson.dropbear.id.au> wrote: > >>>>> > >>>>>> qemu_getrampagesize() works out the minimum host page size backing any > >>>>>> of > >>>>>> guest RAM. This is required in a few places, such as for POWER8 PAPR > >>>>>> KVM > >>>>>> guests, because limitations of the hardware virtualization mean the > >>>>>> guest > >>>>>> can't use pagesizes larger than the host pages backing its memory. > >>>>>> > >>>>>> However, it currently checks against *every* memory backend, whether > >>>>>> or not > >>>>>> it is actually mapped into guest memory at the moment. This is > >>>>>> incorrect. > >>>>>> > >>>>>> This can cause a problem attempting to add memory to a POWER8 pseries > >>>>>> KVM > >>>>>> guest which is configured to allow hugepages in the guest (e.g. > >>>>>> -machine cap-hpt-max-page-size=16m). If you attempt to add > >>>>>> non-hugepage, > >>>>>> you can (correctly) create a memory backend, however it (correctly) > >>>>>> will > >>>>>> throw an error when you attempt to map that memory into the guest by > >>>>>> 'device_add'ing a pc-dimm. > >>>>>> > >>>>>> What's not correct is that if you then reset the guest a startup check > >>>>>> against qemu_getrampagesize() will cause a fatal error because of the > >>>>>> new > >>>>>> memory object, even though it's not mapped into the guest. > >>>>> I'd say that backend should be remove by mgmt app since device_add > >>>>> failed > >>>>> instead of leaving it to hang around. (but fatal error either not a nice > >>>>> behavior on QEMU part) > >>>> > >>>> Indeed, it should be removed. Depending on the options (huge pages with > >>>> prealloc?) memory might be consumed for unused memory. Undesired. > >>> > >>> Right, but if the guest initiates a reboot before the management gets > >>> to that, we'll have a crash. > >>> > >> > >> Yes, I agree. > >> > >> At least on s390x (extending on what Igor said): > >> > >> mc->init() -> s390_memory_init() -> > >> memory_region_allocate_system_memory() -> host_memory_backend_set_mapped() > >> > >> > >> ac->init_machine() -> kvm_arch_init() -> > >> kvm_s390_configure_mempath_backing() -> qemu_getrampagesize() > >> > >> > >> And in vl.c > >> > >> configure_accelerator(current_machine, argv[0]); > > Looking more at it, it is seems s390 is 'broken' anyways. > > We call qemu_getrampagesize() here with huge page backends on CLI > > but memory-backends are initialized later > > qemu_opts_foreach(..., object_create_delayed, ...) > > so s390 doesn't take into account memory backends currently > > BTW that might indeed be true, we only check against --mem-path. It's possible to break it with '-numa node,memdev=...' since we don't really have anything to block that call chain for s390 (but I'd argue it's invalid use of CLI for s390 but it's effectively -mem-path on steroids alternative)
