This patch series improves guest vCPUs performances on Arm during clearing dirty log operations by taking MMU read lock instead of MMU write lock.
vCPUs write protection faults are fixed in Arm using MMU read locks. However, when userspace is clearing dirty logs via KVM_CLEAR_DIRTY_LOG ioctl, then kernel code takes MMU write lock. This will block vCPUs write protection faults and degrade guest performance. This degradation gets worse as guest VM size increases in terms of memory and vCPU count.
In this series, MMU read lock adoption is made possible by using KVM_PGTABLE_WALK_SHARED flag in page walker.
Patches 1 to 5: These patches are modifying dirty_log_perf_test. Intent is to mimic production scenarios where guest keeps on executing while userspace threads collect and clear dirty logs independently.
Three new command line options are added: 1. j: Allows to run guest vCPUs and main thread collecting dirty logs independently of each other after initialization is complete. 2. k: Allows to clear dirty logs in smaller chunks compared to existing whole memslot clear in one call. 3. l: Allows to add customizable wait time between consecutive clear dirty log calls to mimic sending dirty memory to destination.
Patch 7-8: These patches refactor code to move MMU lock operations to arch specific code, refactor Arm's page table walker APIs, and change MMU write lock for clearing dirty logs to read lock. Patch 8 has results showing improvements based on dirty_log_perf_test.
Vipin Sharma (9): KVM: selftests: Allow dirty_log_perf_test to clear dirty memory in chunks KVM: selftests: Add optional delay between consecutive Clear-Dirty-Log calls KVM: selftests: Pass count of read and write accesses from guest to host KVM: selftests: Print read and write accesses of pages by vCPUs in dirty_log_perf_test KVM: selftests: Allow independent execution of vCPUs in dirty_log_perf_test KVM: arm64: Correct the kvm_pgtable_stage2_flush() documentation KVM: mmu: Move mmu lock/unlock to arch code for clear dirty log KMV: arm64: Allow stage2_apply_range_sched() to pass page table walker flags KVM: arm64: Run clear-dirty-log under MMU read lock
arch/arm64/include/asm/kvm_pgtable.h | 17 ++- arch/arm64/kvm/hyp/nvhe/mem_protect.c | 4 +- arch/arm64/kvm/hyp/pgtable.c | 16 ++- arch/arm64/kvm/mmu.c | 36 ++++-- arch/mips/kvm/mmu.c | 2 + arch/riscv/kvm/mmu.c | 2 + arch/x86/kvm/mmu/mmu.c | 3 + .../selftests/kvm/dirty_log_perf_test.c | 108 ++++++++++++++---- .../testing/selftests/kvm/include/memstress.h | 13 ++- tools/testing/selftests/kvm/lib/memstress.c | 43 +++++-- virt/kvm/dirty_ring.c | 2 - virt/kvm/kvm_main.c | 4 - 12 files changed, 185 insertions(+), 65 deletions(-)
base-commit: 95b9779c1758f03cf494e8550d6249a40089ed1c