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]); ... machine_run_board_init() So memory is indeed not mapped before calling qemu_getrampagesize(). We *could* move the call to kvm_s390_configure_mempath_backing() to s390_memory_init(). cap_hpage_1m is not needed before we create VCPUs, so this would work fine. We could than eventually make qemu_getrampagesize() asssert if no backends are mapped at all, to catch other user that rely on this being correct. -- Thanks, David / dhildenb
