* David Hildenbrand (david@xxxxxxxxxx) wrote: > Based-on: 20211011175346.15499-1-david@xxxxxxxxxx > > A virtio-mem device is represented by a single large RAM memory region > backed by a single large mmap. > > Right now, we map that complete memory region into guest physical addres > space, resulting in a very large memory mapping, KVM memory slot, ... > although only a small amount of memory might actually be exposed to the VM. > > For example, when starting a VM with a 1 TiB virtio-mem device that only > exposes little device memory (e.g., 1 GiB) towards the VM initialliy, > in order to hotplug more memory later, we waste a lot of memory on metadata > for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some > optimizations in KVM are being worked on to reduce this metadata overhead > on x86-64 in some cases, it remains a problem with nested VMs and there are > other reasons why we would want to reduce the total memory slot to a > reasonable minimum. > > We want to: > a) Reduce the metadata overhead, including bitmap sizes inside KVM but also > inside QEMU KVM code where possible. > b) Not always expose all device-memory to the VM, to reduce the attack > surface of malicious VMs without using userfaultfd. > > So instead, expose the RAM memory region not by a single large mapping > (consuming one memslot) but instead by multiple mappings, each consuming > one memslot. To do that, we divide the RAM memory region via aliases into > separate parts and only map the aliases into a device container we actually > need. We have to make sure that QEMU won't silently merge the memory > sections corresponding to the aliases (and thereby also memslots), > otherwise we lose atomic updates with KVM and vhost-user, which we deeply > care about when adding/removing memory. Further, to get memslot accounting > right, such merging is better avoided. > > Within the memslots, virtio-mem can (un)plug memory in smaller granularity > dynamically. So memslots are a pure optimization to tackle a) and b) above. > > Memslots are right now mapped once they fall into the usable device region > (which grows/shrinks on demand right now either when requesting to > hotplug more memory or during/after reboots). In the future, with > VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, we'll be able to (un)map aliases even > more dynamically when (un)plugging device blocks. > > > Adding a 500GiB virtio-mem device and not hotplugging any memory results in: > 0000000140000000-000001047fffffff (prio 0, i/o): device-memory > 0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots > > Requesting the VM to consume 2 GiB results in (note: the usable region size > is bigger than 2 GiB, so 3 * 1 GiB memslots are required): > 0000000140000000-000001047fffffff (prio 0, i/o): device-memory > 0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots > 0000000140000000-000000017fffffff (prio 0, ram): alias virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff > 0000000180000000-00000001bfffffff (prio 0, ram): alias virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff > 00000001c0000000-00000001ffffffff (prio 0, ram): alias virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff I've got a vague memory that there were some devices that didn't like doing split IO across a memory region (or something) - some virtio devices? Do you know if that's still true and if that causes a problem? Dave > Requesting the VM to consume 20 GiB results in: > 0000000140000000-000001047fffffff (prio 0, i/o): device-memory > 0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots > 0000000140000000-000000017fffffff (prio 0, ram): alias virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff > 0000000180000000-00000001bfffffff (prio 0, ram): alias virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff > 00000001c0000000-00000001ffffffff (prio 0, ram): alias virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff > 0000000200000000-000000023fffffff (prio 0, ram): alias virtio-mem-memslot-3 @mem0 00000000c0000000-00000000ffffffff > 0000000240000000-000000027fffffff (prio 0, ram): alias virtio-mem-memslot-4 @mem0 0000000100000000-000000013fffffff > 0000000280000000-00000002bfffffff (prio 0, ram): alias virtio-mem-memslot-5 @mem0 0000000140000000-000000017fffffff > 00000002c0000000-00000002ffffffff (prio 0, ram): alias virtio-mem-memslot-6 @mem0 0000000180000000-00000001bfffffff > 0000000300000000-000000033fffffff (prio 0, ram): alias virtio-mem-memslot-7 @mem0 00000001c0000000-00000001ffffffff > 0000000340000000-000000037fffffff (prio 0, ram): alias virtio-mem-memslot-8 @mem0 0000000200000000-000000023fffffff > 0000000380000000-00000003bfffffff (prio 0, ram): alias virtio-mem-memslot-9 @mem0 0000000240000000-000000027fffffff > 00000003c0000000-00000003ffffffff (prio 0, ram): alias virtio-mem-memslot-10 @mem0 0000000280000000-00000002bfffffff > 0000000400000000-000000043fffffff (prio 0, ram): alias virtio-mem-memslot-11 @mem0 00000002c0000000-00000002ffffffff > 0000000440000000-000000047fffffff (prio 0, ram): alias virtio-mem-memslot-12 @mem0 0000000300000000-000000033fffffff > 0000000480000000-00000004bfffffff (prio 0, ram): alias virtio-mem-memslot-13 @mem0 0000000340000000-000000037fffffff > 00000004c0000000-00000004ffffffff (prio 0, ram): alias virtio-mem-memslot-14 @mem0 0000000380000000-00000003bfffffff > 0000000500000000-000000053fffffff (prio 0, ram): alias virtio-mem-memslot-15 @mem0 00000003c0000000-00000003ffffffff > 0000000540000000-000000057fffffff (prio 0, ram): alias virtio-mem-memslot-16 @mem0 0000000400000000-000000043fffffff > 0000000580000000-00000005bfffffff (prio 0, ram): alias virtio-mem-memslot-17 @mem0 0000000440000000-000000047fffffff > 00000005c0000000-00000005ffffffff (prio 0, ram): alias virtio-mem-memslot-18 @mem0 0000000480000000-00000004bfffffff > 0000000600000000-000000063fffffff (prio 0, ram): alias virtio-mem-memslot-19 @mem0 00000004c0000000-00000004ffffffff > 0000000640000000-000000067fffffff (prio 0, ram): alias virtio-mem-memslot-20 @mem0 0000000500000000-000000053fffffff > > Requesting the VM to consume 5 GiB and rebooting (note: usable region size > will change during reboots) results in: > 0000000140000000-000001047fffffff (prio 0, i/o): device-memory > 0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots > 0000000140000000-000000017fffffff (prio 0, ram): alias virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff > 0000000180000000-00000001bfffffff (prio 0, ram): alias virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff > 00000001c0000000-00000001ffffffff (prio 0, ram): alias virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff > 0000000200000000-000000023fffffff (prio 0, ram): alias virtio-mem-memslot-3 @mem0 00000000c0000000-00000000ffffffff > 0000000240000000-000000027fffffff (prio 0, ram): alias virtio-mem-memslot-4 @mem0 0000000100000000-000000013fffffff > 0000000280000000-00000002bfffffff (prio 0, ram): alias virtio-mem-memslot-5 @mem0 0000000140000000-000000017fffffff > > > In addition to other factors, we limit the number of memslots to 1024 per > devices and the size of one memslot to at least 1 GiB. So only a 1 TiB > virtio-mem device could consume 1024 memslots in the "worst" case. To > calculate a memslot limit for a device, we use a heuristic based on all > available memslots for memory devices and the percentage of > "device size":"total memory device area size". Users can further limit > the maximum number of memslots that will be used by a device by setting > the "max-memslots" property. It's expected to be set to "0" (auto) in most > setups. > > In recent setups (e.g., KVM with ~32k memslots, vhost-user with ~4k > memslots after this series), we'll get the biggest benefit. In special > setups (e.g., older KVM, vhost kernel with 64 memslots), we'll get some > benefit -- the individual memslots will be bigger. > > Future work: > - vhost-user and libvhost-user optimizations for handling more memslots > more efficiently. > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Eduardo Habkost <ehabkost@xxxxxxxxxx> > Cc: Marcel Apfelbaum <marcel.apfelbaum@xxxxxxxxx> > Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx> > Cc: Igor Mammedov <imammedo@xxxxxxxxxx> > Cc: Ani Sinha <ani@xxxxxxxxxxx> > Cc: Peter Xu <peterx@xxxxxxxxxx> > Cc: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> > Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx> > Cc: Richard Henderson <richard.henderson@xxxxxxxxxx> > Cc: Philippe Mathieu-Daudé <f4bug@xxxxxxxxx> > Cc: kvm@xxxxxxxxxxxxxxx > > David Hildenbrand (15): > memory: Drop mapping check from > memory_region_get_ram_discard_manager() > kvm: Return number of free memslots > vhost: Return number of free memslots > memory: Allow for marking memory region aliases unmergeable > vhost: Don't merge unmergeable memory sections > memory-device: Move memory_device_check_addable() directly into > memory_device_pre_plug() > memory-device: Generalize memory_device_used_region_size() > memory-device: Support memory devices that consume a variable number > of memslots > vhost: Respect reserved memslots for memory devices when realizing a > vhost device > virtio-mem: Set the RamDiscardManager for the RAM memory region > earlier > virtio-mem: Fix typo in virito_mem_intersect_memory_section() function > name > virtio-mem: Expose device memory via separate memslots > vhost-user: Increase VHOST_USER_MAX_RAM_SLOTS to 496 with > CONFIG_VIRTIO_MEM > libvhost-user: Increase VHOST_USER_MAX_RAM_SLOTS to 4096 > virtio-mem: Set "max-memslots" to 0 (auto) for the 6.2 machine > > accel/kvm/kvm-all.c | 24 ++- > accel/stubs/kvm-stub.c | 4 +- > hw/core/machine.c | 1 + > hw/mem/memory-device.c | 167 +++++++++++++++--- > hw/virtio/vhost-stub.c | 2 +- > hw/virtio/vhost-user.c | 7 +- > hw/virtio/vhost.c | 17 +- > hw/virtio/virtio-mem-pci.c | 22 +++ > hw/virtio/virtio-mem.c | 202 ++++++++++++++++++++-- > include/exec/memory.h | 23 +++ > include/hw/mem/memory-device.h | 32 ++++ > include/hw/virtio/vhost.h | 2 +- > include/hw/virtio/virtio-mem.h | 29 +++- > include/sysemu/kvm.h | 2 +- > softmmu/memory.c | 35 +++- > stubs/qmp_memory_device.c | 5 + > subprojects/libvhost-user/libvhost-user.h | 7 +- > 17 files changed, 499 insertions(+), 82 deletions(-) > > -- > 2.31.1 > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
