| Message ID | 20230908142136.403541-1-david@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | virtio-mem: Expose device memory through multiple memslots | expand |
@MST, any comment on the vhost bits (mostly uncontroversial and only in the memslot domain)? I'm planning on queuing this myself (but will wait a bit more), unless you want to take it. On 08.09.23 16:21, David Hildenbrand wrote: > Quoting from patch #14: > > Having large virtio-mem devices that only expose little memory to a VM > is currently a problem: we map the whole sparse memory region into the > guest using a single memslot, resulting in one gigantic memslot in KVM. > KVM allocates metadata for the whole memslot, which can result in quite > some memory waste. > > Assuming we have a 1 TiB virtio-mem device and only expose little (e.g., > 1 GiB) memory, we would create a single 1 TiB memslot and KVM has to > allocate metadata for that 1 TiB memslot: on x86, this implies allocating > a significant amount of memory for metadata: > > (1) RMAP: 8 bytes per 4 KiB, 8 bytes per 2 MiB, 8 bytes per 1 GiB > -> For 1 TiB: 2147483648 + 4194304 + 8192 = ~ 2 GiB (0.2 %) > > With the TDP MMU (cat /sys/module/kvm/parameters/tdp_mmu) this gets > allocated lazily when required for nested VMs > (2) gfn_track: 2 bytes per 4 KiB > -> For 1 TiB: 536870912 = ~512 MiB (0.05 %) > (3) lpage_info: 4 bytes per 2 MiB, 4 bytes per 1 GiB > -> For 1 TiB: 2097152 + 4096 = ~2 MiB (0.0002 %) > (4) 2x dirty bitmaps for tracking: 2x 1 bit per 4 KiB page > -> For 1 TiB: 536870912 = 64 MiB (0.006 %) > > So we primarily care about (1) and (2). The bad thing is, that the > memory consumption doubles once SMM is enabled, because we create the > memslot once for !SMM and once for SMM. > > Having a 1 TiB memslot without the TDP MMU consumes around: > * With SMM: 5 GiB > * Without SMM: 2.5 GiB > Having a 1 TiB memslot with the TDP MMU consumes around: > * With SMM: 1 GiB > * Without SMM: 512 MiB > > ... and that's really something we want to optimize, to be able to just > start a VM with small boot memory (e.g., 4 GiB) and a virtio-mem device > that can grow very large (e.g., 1 TiB). > > Consequently, using multiple memslots and only mapping the memslots we > really need can significantly reduce memory waste and speed up > memslot-related operations. Let's expose the sparse RAM memory region using > multiple memslots, mapping only the memslots we currently need into our > device memory region container. > > The hyper-v balloon driver has similar demands [1]. > > For virtio-mem, this has to be turned manually on ("multiple-memslots=on"), > due to the interaction with vhost (below). > > If we have less than 509 memslots available, we always default to a single > memslot. Otherwise, we automatically decide how many memslots to use > based on a simple heuristic (see patch #12), and try not to use more than > 256 memslots across all memory devices: our historical DIMM limit. > > As soon as any memory devices automatically decided on using more than > one memslot, vhost devices that support less than 509 memslots (e.g., > currently most vhost-user devices like with virtiofsd) can no longer be > plugged as a precaution. > > Quoting from patch #12: > > Plugging vhost devices with less than 509 memslots available while we > have memory devices plugged that consume multiple memslots due to > automatic decisions can be problematic. Most configurations might just fail > due to "limit < used + reserved", however, it can also happen that these > memory devices would suddenly consume memslots that would actually be > required by other memslot consumers (boot, PCI BARs) later. Note that this > has always been sketchy with vhost devices that support only a small number > of memslots; but we don't want to make it any worse.So let's keep it simple > and simply reject plugging such vhost devices in such a configuration. > > Eventually, all vhost devices that want to be fully compatible with such > memory devices should support a decent number of memslots (>= 509). > > > The recommendation is to plug such vhost devices before the virtio-mem > decides, or to not set "multiple-memslots=on". As soon as these devices > support a reasonable number of memslots (>= 509), this will start working > automatically. > > I run some tests on x86_64, now also including vfio tests. Seems to work > as expected, even when multiple memslots are used. > > > Patch #1 -- #3 are from [2] that were not picked up yet. > > Patch #4 -- #12 add handling of multiple memslots to memory devices > > Patch #13 -- #14 add "multiple-memslots=on" support to virtio-mem > > Patch #15 -- #16 make sure that virtio-mem memslots can be enabled/disable > atomically > > v2 -> v3: > * "kvm: Return number of free memslots" > -> Return 0 in stub > * "kvm: Add stub for kvm_get_max_memslots()" > -> Return 0 in stub > * Adjust other patches to check for kvm_enabled() before calling > kvm_get_free_memslots()/kvm_get_max_memslots() > * Add RBs > > v1 -> v2: > * Include patches from [1] > * A lot of code simplification and reorganization, too many to spell out > * don't add a general soft-limit on memslots, to avoid warning in sane > setups > * Simplify handling of vhost devices with a small number of memslots: > simply fail plugging them > * "virtio-mem: Expose device memory via multiple memslots if enabled" > -> Fix one "is this the last memslot" check > * Much more testing > > > [1] https://lkml.kernel.org/r/cover.1689786474.git.maciej.szmigiero@oracle.com > [2] https://lkml.kernel.org/r/20230523185915.540373-1-david@redhat.com > > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Igor Mammedov <imammedo@redhat.com> > Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com> > Cc: "Michael S. Tsirkin" <mst@redhat.com> > Cc: Peter Xu <peterx@redhat.com> > Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org> > Cc: Eduardo Habkost <eduardo@habkost.net> > Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com> > Cc: Yanan Wang <wangyanan55@huawei.com> > Cc: Michal Privoznik <mprivozn@redhat.com> > Cc: Daniel P. Berrangé <berrange@redhat.com> > Cc: Gavin Shan <gshan@redhat.com> > Cc: Alex Williamson <alex.williamson@redhat.com> > Cc: Stefan Hajnoczi <stefanha@redhat.com> > Cc: Maciej S. Szmigiero <mail@maciej.szmigiero.name> > Cc: kvm@vger.kernel.org > > David Hildenbrand (16): > vhost: Rework memslot filtering and fix "used_memslot" tracking > vhost: Remove vhost_backend_can_merge() callback > softmmu/physmem: Fixup qemu_ram_block_from_host() documentation > kvm: Return number of free memslots > vhost: Return number of free memslots > memory-device: Support memory devices with multiple memslots > stubs: Rename qmp_memory_device.c to memory_device.c > memory-device: Track required and actually used memslots in > DeviceMemoryState > memory-device,vhost: Support memory devices that dynamically consume > memslots > kvm: Add stub for kvm_get_max_memslots() > vhost: Add vhost_get_max_memslots() > memory-device,vhost: Support automatic decision on the number of > memslots > memory: Clarify mapping requirements for RamDiscardManager > virtio-mem: Expose device memory via multiple memslots if enabled > memory,vhost: Allow for marking memory device memory regions > unmergeable > virtio-mem: Mark memslot alias memory regions unmergeable > > MAINTAINERS | 1 + > accel/kvm/kvm-all.c | 35 ++- > accel/stubs/kvm-stub.c | 9 +- > hw/mem/memory-device.c | 196 ++++++++++++- > hw/virtio/vhost-stub.c | 9 +- > hw/virtio/vhost-user.c | 21 +- > hw/virtio/vhost-vdpa.c | 1 - > hw/virtio/vhost.c | 103 +++++-- > hw/virtio/virtio-mem-pci.c | 21 ++ > hw/virtio/virtio-mem.c | 272 +++++++++++++++++- > include/exec/cpu-common.h | 15 + > include/exec/memory.h | 27 +- > include/hw/boards.h | 14 +- > include/hw/mem/memory-device.h | 57 ++++ > include/hw/virtio/vhost-backend.h | 9 +- > include/hw/virtio/vhost.h | 3 +- > include/hw/virtio/virtio-mem.h | 23 +- > include/sysemu/kvm.h | 4 +- > include/sysemu/kvm_int.h | 1 + > softmmu/memory.c | 35 ++- > softmmu/physmem.c | 17 -- > .../{qmp_memory_device.c => memory_device.c} | 10 + > stubs/meson.build | 2 +- > 23 files changed, 779 insertions(+), 106 deletions(-) > rename stubs/{qmp_memory_device.c => memory_device.c} (56%) >
On 11.09.23 09:45, David Hildenbrand wrote: > @MST, any comment on the vhost bits (mostly uncontroversial and only in > the memslot domain)? > > I'm planning on queuing this myself (but will wait a bit more), unless > you want to take it. I'm queuing this to https://github.com/davidhildenbrand/qemu.git mem-next and plan on sending a PULL request on Friday. So if anybody has objections, please let me know ASAP :)
On 19.09.23 10:20, David Hildenbrand wrote: > On 11.09.23 09:45, David Hildenbrand wrote: >> @MST, any comment on the vhost bits (mostly uncontroversial and only in >> the memslot domain)? >> >> I'm planning on queuing this myself (but will wait a bit more), unless >> you want to take it. > > I'm queuing this to > > https://github.com/davidhildenbrand/qemu.git mem-next > > and plan on sending a PULL request on Friday. > > So if anybody has objections, please let me know ASAP :) > .... and I dropped it again after realizing that migration needs care (activate memslots on migrationd estination). I'll look into that and resend once that is fixed.
Quoting from patch #14: Having large virtio-mem devices that only expose little memory to a VM is currently a problem: we map the whole sparse memory region into the guest using a single memslot, resulting in one gigantic memslot in KVM. KVM allocates metadata for the whole memslot, which can result in quite some memory waste. Assuming we have a 1 TiB virtio-mem device and only expose little (e.g., 1 GiB) memory, we would create a single 1 TiB memslot and KVM has to allocate metadata for that 1 TiB memslot: on x86, this implies allocating a significant amount of memory for metadata: (1) RMAP: 8 bytes per 4 KiB, 8 bytes per 2 MiB, 8 bytes per 1 GiB -> For 1 TiB: 2147483648 + 4194304 + 8192 = ~ 2 GiB (0.2 %) With the TDP MMU (cat /sys/module/kvm/parameters/tdp_mmu) this gets allocated lazily when required for nested VMs (2) gfn_track: 2 bytes per 4 KiB -> For 1 TiB: 536870912 = ~512 MiB (0.05 %) (3) lpage_info: 4 bytes per 2 MiB, 4 bytes per 1 GiB -> For 1 TiB: 2097152 + 4096 = ~2 MiB (0.0002 %) (4) 2x dirty bitmaps for tracking: 2x 1 bit per 4 KiB page -> For 1 TiB: 536870912 = 64 MiB (0.006 %) So we primarily care about (1) and (2). The bad thing is, that the memory consumption doubles once SMM is enabled, because we create the memslot once for !SMM and once for SMM. Having a 1 TiB memslot without the TDP MMU consumes around: * With SMM: 5 GiB * Without SMM: 2.5 GiB Having a 1 TiB memslot with the TDP MMU consumes around: * With SMM: 1 GiB * Without SMM: 512 MiB ... and that's really something we want to optimize, to be able to just start a VM with small boot memory (e.g., 4 GiB) and a virtio-mem device that can grow very large (e.g., 1 TiB). Consequently, using multiple memslots and only mapping the memslots we really need can significantly reduce memory waste and speed up memslot-related operations. Let's expose the sparse RAM memory region using multiple memslots, mapping only the memslots we currently need into our device memory region container. The hyper-v balloon driver has similar demands [1]. For virtio-mem, this has to be turned manually on ("multiple-memslots=on"), due to the interaction with vhost (below). If we have less than 509 memslots available, we always default to a single memslot. Otherwise, we automatically decide how many memslots to use based on a simple heuristic (see patch #12), and try not to use more than 256 memslots across all memory devices: our historical DIMM limit. As soon as any memory devices automatically decided on using more than one memslot, vhost devices that support less than 509 memslots (e.g., currently most vhost-user devices like with virtiofsd) can no longer be plugged as a precaution. Quoting from patch #12: Plugging vhost devices with less than 509 memslots available while we have memory devices plugged that consume multiple memslots due to automatic decisions can be problematic. Most configurations might just fail due to "limit < used + reserved", however, it can also happen that these memory devices would suddenly consume memslots that would actually be required by other memslot consumers (boot, PCI BARs) later. Note that this has always been sketchy with vhost devices that support only a small number of memslots; but we don't want to make it any worse.So let's keep it simple and simply reject plugging such vhost devices in such a configuration. Eventually, all vhost devices that want to be fully compatible with such memory devices should support a decent number of memslots (>= 509). The recommendation is to plug such vhost devices before the virtio-mem decides, or to not set "multiple-memslots=on". As soon as these devices support a reasonable number of memslots (>= 509), this will start working automatically. I run some tests on x86_64, now also including vfio tests. Seems to work as expected, even when multiple memslots are used. Patch #1 -- #3 are from [2] that were not picked up yet. Patch #4 -- #12 add handling of multiple memslots to memory devices Patch #13 -- #14 add "multiple-memslots=on" support to virtio-mem Patch #15 -- #16 make sure that virtio-mem memslots can be enabled/disable atomically v2 -> v3: * "kvm: Return number of free memslots" -> Return 0 in stub * "kvm: Add stub for kvm_get_max_memslots()" -> Return 0 in stub * Adjust other patches to check for kvm_enabled() before calling kvm_get_free_memslots()/kvm_get_max_memslots() * Add RBs v1 -> v2: * Include patches from [1] * A lot of code simplification and reorganization, too many to spell out * don't add a general soft-limit on memslots, to avoid warning in sane setups * Simplify handling of vhost devices with a small number of memslots: simply fail plugging them * "virtio-mem: Expose device memory via multiple memslots if enabled" -> Fix one "is this the last memslot" check * Much more testing [1] https://lkml.kernel.org/r/cover.1689786474.git.maciej.szmigiero@oracle.com [2] https://lkml.kernel.org/r/20230523185915.540373-1-david@redhat.com Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Igor Mammedov <imammedo@redhat.com> Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com> Cc: "Michael S. Tsirkin" <mst@redhat.com> Cc: Peter Xu <peterx@redhat.com> Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org> Cc: Eduardo Habkost <eduardo@habkost.net> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com> Cc: Yanan Wang <wangyanan55@huawei.com> Cc: Michal Privoznik <mprivozn@redhat.com> Cc: Daniel P. Berrangé <berrange@redhat.com> Cc: Gavin Shan <gshan@redhat.com> Cc: Alex Williamson <alex.williamson@redhat.com> Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Maciej S. Szmigiero <mail@maciej.szmigiero.name> Cc: kvm@vger.kernel.org David Hildenbrand (16): vhost: Rework memslot filtering and fix "used_memslot" tracking vhost: Remove vhost_backend_can_merge() callback softmmu/physmem: Fixup qemu_ram_block_from_host() documentation kvm: Return number of free memslots vhost: Return number of free memslots memory-device: Support memory devices with multiple memslots stubs: Rename qmp_memory_device.c to memory_device.c memory-device: Track required and actually used memslots in DeviceMemoryState memory-device,vhost: Support memory devices that dynamically consume memslots kvm: Add stub for kvm_get_max_memslots() vhost: Add vhost_get_max_memslots() memory-device,vhost: Support automatic decision on the number of memslots memory: Clarify mapping requirements for RamDiscardManager virtio-mem: Expose device memory via multiple memslots if enabled memory,vhost: Allow for marking memory device memory regions unmergeable virtio-mem: Mark memslot alias memory regions unmergeable MAINTAINERS | 1 + accel/kvm/kvm-all.c | 35 ++- accel/stubs/kvm-stub.c | 9 +- hw/mem/memory-device.c | 196 ++++++++++++- hw/virtio/vhost-stub.c | 9 +- hw/virtio/vhost-user.c | 21 +- hw/virtio/vhost-vdpa.c | 1 - hw/virtio/vhost.c | 103 +++++-- hw/virtio/virtio-mem-pci.c | 21 ++ hw/virtio/virtio-mem.c | 272 +++++++++++++++++- include/exec/cpu-common.h | 15 + include/exec/memory.h | 27 +- include/hw/boards.h | 14 +- include/hw/mem/memory-device.h | 57 ++++ include/hw/virtio/vhost-backend.h | 9 +- include/hw/virtio/vhost.h | 3 +- include/hw/virtio/virtio-mem.h | 23 +- include/sysemu/kvm.h | 4 +- include/sysemu/kvm_int.h | 1 + softmmu/memory.c | 35 ++- softmmu/physmem.c | 17 -- .../{qmp_memory_device.c => memory_device.c} | 10 + stubs/meson.build | 2 +- 23 files changed, 779 insertions(+), 106 deletions(-) rename stubs/{qmp_memory_device.c => memory_device.c} (56%)