A virtio-mem device manages a memory region in guest physical address
space, represented as a single (currently large) memory region in QEMU,
mapped into system memory address space. Before the guest is allowed to use
memory blocks, it must coordinate with the hypervisor (plug blocks). After
a reboot, all memory is usually unplugged - when the guest comes up, it
detects the virtio-mem device and selects memory blocks to plug (based on
resize requests from the hypervisor).
Memory hot(un)plug consists of (un)plugging memory blocks via a virtio-mem
device (triggered by the guest). When unplugging blocks, we discard the
memory - similar to memory balloon inflation. In contrast to memory
ballooning, we always know which memory blocks a guest may actually use -
especially during a reboot, after a crash, or after kexec (and during
hibernation as well). Guests agreed to not access unplugged memory again,
especially not via DMA.
The issue with vfio is, that it cannot deal with random discards - for this
reason, virtio-mem and vfio can currently only run mutually exclusive.
Especially, vfio would currently map the whole memory region (with possible
only little/no plugged blocks), resulting in all pages getting pinned and
therefore resulting in a higher memory consumption than expected (turning
virtio-mem basically useless in these environments).
To make vfio work nicely with virtio-mem, we have to map only the plugged
blocks, and map/unmap properly when plugging/unplugging blocks (including
discarding of RAM when unplugging). We achieve that by using a new notifier
mechanism that communicates changes.
It's important to map memory in the granularity in which we could see
unmaps again (-> virtio-mem block size) - so when e.g., plugging
consecutive 100 MB with a block size of 2 MB, we need 50 mappings. When
unmapping, we can use a single vfio_unmap call for the applicable range.
We expect that the block size of virtio-mem devices will be fairly large
in the future (to not run out of mappings and to improve hot(un)plug
performance), configured by the user, when used with vfio (e.g., 128MB,
1G, ...), but it will depend on the setup.
More info regarding virtio-mem can be found at:
https://virtio-mem.gitlab.io/
v7 is located at:
git@github.com:davidhildenbrand/qemu.git virtio-mem-vfio-v7
v6 -> v7:
- s/RamDiscardMgr/RamDiscardManager/
- "memory: Introduce RamDiscardManager for RAM memory regions"
-- Make RamDiscardManager/RamDiscardListener eat MemoryRegionSections
-- Replace notify_discard_all callback by double_discard_supported
-- Reshuffle the individual hunks in memory.h
-- Provide function wrappers for RamDiscardManager calls
- "memory: Helpers to copy/free a MemoryRegionSection"
-- Added
- "virtio-mem: Implement RamDiscardManager interface"
-- Work on MemoryRegionSections instead of ranges
-- Minor optimizations
- "vfio: Support for RamDiscardManager in the !vIOMMU case"
-- Simplify based on new interfaces / MemoryRegionSections
-- Minor cleanups and optimizations
-- Add a comment regarding dirty bitmap sync.
-- Don't store "offset_within_region" in VFIORamDiscardListener
- "vfio: Support for RamDiscardManager in the vIOMMU case"
-- Adjust to new interface
- "softmmu/physmem: Don't use atomic operations in ..."
-- Rename variables
- "softmmu/physmem: Extend ram_block_discard_(require|disable) ..."
-- Rename variables
- Rebased and retested
v5 -> v6:
- "memory: Introduce RamDiscardMgr for RAM memory regions"
-- Fix variable names in one prototype.
- "virtio-mem: Don't report errors when ram_block_discard_range() fails"
-- Added
- "virtio-mem: Implement RamDiscardMgr interface"
-- Don't report an error if discarding fails
- Rebased and retested
v4 -> v5:
- "vfio: Support for RamDiscardMgr in the !vIOMMU case"
-- Added more assertions for granularity vs. iommu supported pagesize
- "vfio: Sanity check maximum number of DMA mappings with RamDiscardMgr"
-- Fix accounting of mappings
- "vfio: Disable only uncoordinated discards for VFIO_TYPE1 iommus"
-- Fence off SPAPR and add some comments regarding future support.
-- Tweak patch description
- Rebase and retest
v3 -> v4:
- "vfio: Query and store the maximum number of DMA mappings
-- Limit the patch to querying and storing only
-- Renamed to "vfio: Query and store the maximum number of possible DMA
mappings"
- "vfio: Support for RamDiscardMgr in the !vIOMMU case"
-- Remove sanity checks / warning the user
- "vfio: Sanity check maximum number of DMA mappings with RamDiscardMgr"
-- Perform sanity checks by looking at the number of memslots and all
registered RamDiscardMgr sections
- Rebase and retest
- Reshuffled the patches slightly
v2 -> v3:
- Rebased + retested
- Fixed some typos
- Added RB's
v1 -> v2:
- "memory: Introduce RamDiscardMgr for RAM memory regions"
-- Fix some errors in the documentation
-- Make register_listener() notify about populated parts and
unregister_listener() notify about discarding populated parts, to
simplify future locking inside virtio-mem, when handling requests via a
separate thread.
- "vfio: Query and store the maximum number of DMA mappings"
-- Query number of mappings and track mappings (except for vIOMMU)
- "vfio: Support for RamDiscardMgr in the !vIOMMU case"
-- Adapt to RamDiscardMgr changes and warn via generic DMA reservation
- "vfio: Support for RamDiscardMgr in the vIOMMU case"
-- Use vmstate priority to handle migration dependencies
RFC - v1:
- VFIO migration code. Due to missing kernel support, I cannot really test
if that part works.
- Understand/test/document vIOMMU implications, also regarding migration
- Nicer ram_block_discard_disable/require handling.
- s/SparseRAMHandler/RamDiscardMgr/, refactorings, cleanups, documentation,
testing, ...
David Hildenbrand (13):
memory: Introduce RamDiscardManager for RAM memory regions
memory: Helpers to copy/free a MemoryRegionSection
virtio-mem: Factor out traversing unplugged ranges
virtio-mem: Don't report errors when ram_block_discard_range() fails
virtio-mem: Implement RamDiscardManager interface
vfio: Support for RamDiscardManager in the !vIOMMU case
vfio: Query and store the maximum number of possible DMA mappings
vfio: Sanity check maximum number of DMA mappings with
RamDiscardManager
vfio: Support for RamDiscardManager in the vIOMMU case
softmmu/physmem: Don't use atomic operations in
ram_block_discard_(disable|require)
softmmu/physmem: Extend ram_block_discard_(require|disable) by two
discard types
virtio-mem: Require only coordinated discards
vfio: Disable only uncoordinated discards for VFIO_TYPE1 iommus
hw/vfio/common.c | 315 +++++++++++++++++++++++++-
hw/virtio/virtio-mem.c | 391 ++++++++++++++++++++++++++++-----
include/exec/memory.h | 324 +++++++++++++++++++++++++--
include/hw/vfio/vfio-common.h | 12 +
include/hw/virtio/virtio-mem.h | 3 +
include/migration/vmstate.h | 1 +
softmmu/memory.c | 98 +++++++++
softmmu/physmem.c | 108 ++++++---
8 files changed, 1133 insertions(+), 119 deletions(-)