On Mon, 29 Jul 2019 10:52:27 -0400 Igor Mammedov <imamm...@redhat.com> wrote:
cc:ing some folks for awareness. > While looking into unifying guest RAM allocation to use hostmem backends > for initial RAM (especially when -mempath is used) and retiring > memory_region_allocate_system_memory() API, leaving only single hostmem > backend, > I was inspecting how currently it is used by boards and it turns out several > boards abuse it by calling the function several times (despite documented > contract > forbiding it). > > s390 is one of such boards where KVM limitation on memslot size got propagated > to board design and memory_region_allocate_system_memory() was abused to > satisfy > KVM requirement for max RAM chunk where memory region alias would suffice. > > Unfortunately, memory_region_allocate_system_memory() usage created migration > dependency where guest RAM is transferred in migration stream as several > RAMBlocks > if it's more than KVM_SLOT_MAX_BYTES. > > In order to replace these several RAM chunks with a single memdev and keep it > working with KVM memslot size limit and migration compatible, following was > done: > * [2/2] use memory region aliases to partition hostmem backend RAM on > KVM_SLOT_MAX_BYTES chunks, which should keep KVM side working > * [1/2] hacked memory region aliases (to ram memory regions only) to have > its own RAMBlocks pointing to RAM chunks owned by aliased memory > region. While it's admittedly a hack, but it's relatively simple > and > allows board code rashape migration stream as necessary > > I haven't tried to use migratable aliases on x86 machines, but > with it > it could be possible to drop legacy RAM allocation and compat knob > (cd5ff8333a) dropping '-numa node,mem' completely even for old > machines. > > PS: > Tested with ping pong cross version migration on s390 machine > (with reduced KVM_SLOT_MAX_BYTES since I don't have access to large > enough host) > > > Igor Mammedov (2): > memory: make MemoryRegion alias migratable > s390: do not call memory_region_allocate_system_memory() multiple > times > > exec.c | 7 ++++--- > hw/s390x/s390-virtio-ccw.c | 20 +++++++++++++++----- > memory.c | 5 +++++ > 3 files changed, 24 insertions(+), 8 deletions(-) >
