Presently KVM only takes a read lock for stage 2 faults if it believes the fault can be fixed by relaxing permissions on a PTE (write unprotect for dirty logging). Otherwise, stage 2 faults grab the write lock, which predictably can pile up all the vCPUs in a sufficiently large VM. Like the TDP MMU for x86, this series loosens the locking around manipulations of the stage 2 page tables to allow parallel faults. RCU and atomics are exploited to safely build/destroy the stage 2 page tables in light of multiple software observers. Patches 1-4 clean up the context associated with a page table walk / PTE visit. This is helpful for: - Extending the context passed through for a visit - Building page table walkers that operate outside of a kvm_pgtable context (e.g. RCU callback) Patches 5-6 clean up the stage-2 map walkers by calling a helper to tear down removed tables. There is a small improvement here in that a broken PTE is replaced more quickly, as page table teardown happens afterwards. Patches 7-9 sprinkle in RCU to the page table walkers, punting the teardown of removed tables to an RCU callback. Patches 10-14 implement the meat of this series, extending the 'break-before-make' sequence with atomics to realize locking on PTEs. Effectively a cmpxchg() is used to 'break' a PTE, thereby serializing changes to a given PTE. Finally, patch 15 flips the switch on all the new code and starts grabbing the read side of the MMU lock for stage 2 faults. Applies to 6.1-rc2. Tested with KVM selftests, kvm-unit-tests, and Google's internal VMM (Vanadium). Also tested with lockdep enabled and saw no puke for RCU. Branch available at: https://github.com/oupton/linux kvm-arm64/parallel_mmu benchmarked with dirty_log_perf_test, scaling from 1 to 48 vCPUs with 4GB of memory per vCPU backed by THP. ./dirty_log_perf_test -s anonymous_thp -m 2 -b 4G -v ${NR_VCPUS} Time to dirty memory: +-------+----------+-------------------+ | vCPUs | 6.1-rc2 | 6.1-rc2 + series | +-------+----------+-------------------+ | 1 | 0.87s | 0.93s | | 2 | 1.11s | 1.16s | | 4 | 2.39s | 1.27s | | 8 | 5.01s | 1.39s | | 16 | 8.89s | 2.07s | | 32 | 19.90s | 4.45s | | 48 | 32.10s | 6.23s | +-------+----------+-------------------+ It is also worth mentioning that the time to populate memory has improved: +-------+----------+-------------------+ | vCPUs | 6.1-rc2 | 6.1-rc2 + series | +-------+----------+-------------------+ | 1 | 0.21s | 0.17s | | 2 | 0.26s | 0.23s | | 4 | 0.39s | 0.31s | | 8 | 0.68s | 0.39s | | 16 | 1.26s | 0.53s | | 32 | 2.51s | 1.04s | | 48 | 3.94s | 1.55s | +-------+----------+-------------------+ v2 -> v3: - Drop const qualifier from opaque argument pointer. The whole visitor context is passed as a const pointer. - kvm_set_spte_gfn() is called under the write lock; don't set the SHARED bit in this case (Sean). - Fix build warning resulting from reparameterization residue (test robot). - Add an assertion that the RCU read lock is held before the raw pointer is used in visitors (Marc, off list). v1 -> v2: - It builds! :-) - Roll all of the context associated with PTE visit into a stack-allocated structure - Clean up the oddball handling of PTE values, avoiding a UAF along the way (Quentin) - Leave the re-reading of the PTE after WALK_LEAF in place instead of attempting to return the installed PTE value (David) - Mention why RCU is stubbed out for hyp page table walkers (David) - Ensure that all reads of page table memory pass through an RCU-protected pointer. The lifetime of the dereference is contained within __kvm_pgtable_visit() (David). - Ensure that no user of stage2_map_walker() passes TABLE_POST (David) - Unwire the page table walkers from relying on struct kvm_pgtable, simplifying the passed context to RCU callbacks. - Key rcu_dereference() off of a page table flag indicating a shared walk. This is clear when either (1) the write lock is held or (2) called from an RCU callback. v1: https://lore.kernel.org/kvmarm/20220830194132.962932-1-oliver.upton@xxxxxxxxx/ v2: https://lore.kernel.org/kvmarm/20221007232818.459650-1-oliver.upton@xxxxxxxxx/ Oliver Upton (15): KVM: arm64: Combine visitor arguments into a context structure KVM: arm64: Stash observed pte value in visitor context KVM: arm64: Pass mm_ops through the visitor context KVM: arm64: Don't pass kvm_pgtable through kvm_pgtable_walk_data KVM: arm64: Add a helper to tear down unlinked stage-2 subtrees KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make KVM: arm64: Use an opaque type for pteps KVM: arm64: Protect stage-2 traversal with RCU KVM: arm64: Free removed stage-2 tables in RCU callback KVM: arm64: Atomically update stage 2 leaf attributes in parallel walks KVM: arm64: Split init and set for table PTE KVM: arm64: Make block->table PTE changes parallel-aware KVM: arm64: Make leaf->leaf PTE changes parallel-aware KVM: arm64: Make table->block changes parallel-aware KVM: arm64: Handle stage-2 faults in parallel arch/arm64/include/asm/kvm_pgtable.h | 92 +++- arch/arm64/kvm/hyp/nvhe/mem_protect.c | 21 +- arch/arm64/kvm/hyp/nvhe/setup.c | 22 +- arch/arm64/kvm/hyp/pgtable.c | 624 ++++++++++++++------------ arch/arm64/kvm/mmu.c | 51 ++- 5 files changed, 463 insertions(+), 347 deletions(-) base-commit: 247f34f7b80357943234f93f247a1ae6b6c3a740 -- 2.38.1.273.g43a17bfeac-goog
