There are two stages of page fault. The guest kernel is responsible for handling stage-1 page fault, while the host kernel is to take care of the stage-2 page fault. When the guest is trapped to host because of stage-2 page fault, the guest is suspended until the requested memory (page) is populated. Sometimes, the cost to populate the requested page isn't cheap and can take hundreds of milliseconds in extreme cases. Similarly, the guest has to wait until the requested memory is ready in the scenario of post-copy live migration. This series introduces the feature (Asynchronous Page Fault) to improve situation, so that the guest needn't have to wait in the scenarios. With it, the overall performance is improved on the guest. This series depends on the feature "SDEI virtualization" and QEMU changes. All code changes can be found from github: https://github.com/gwshan/linux ("kvm/arm64_sdei") # SDEI virtualization https://github.com/gwshan/linux ("kvm/arm64_apf") # This series + "sdei" https://github.com/gwshan/qemu ("kvm/arm64_apf") # QEMU code changes About the design, the details can be found from last patch. Generally, it's driven by two notifications: page-not-present and page-ready. They are delivered from the host to guest via SDEI event and PPI separately. In the mean while, each notification is always associated with a token, used to identify the notification. The token is passed by the shared memory between host/guest. Besides, the SMCCC and ioctl interface are mitigated by VMM and guest to configure, enable, disable, even migrate the functionality. When the guest is trapped to host because of stage-2 page fault, a page-not-present notification is raised by the host, and sent to the guest through dedicated SDEI event (0x40400001) if the requested page can't be populated immediately. In the mean while, a (background) worker is also started to populate the requested page. On receiving the SDEI event, the guest marks the current running process with special flag (TIF_ASYNC_PF) and associates it with a pre-allocated waitqueue. At same time, a (reschedule) IPI is sent to current CPU. After the SDEI event is acknowledged by the guest, the (reschedule) IPI is delivered and it causes context switch from that process tagged with TIF_ASYNC_PF to another process. Later on, a page-ready notification is sent to guest after the requested page is populated by the (background) worker. On receiving the interrupt, the guest uses the associated token to locate the process, which was previously suspended because of page-not-present. The flag (TIF_ASYNC_PF) is cleared for the suspended process and it's waken up. The series is organized as below: PATCH[01-04] makes the GFN hash table management generic so that it can be shared by x86/arm64. PATCH[05-06] preparatory work to support asynchronous page fault. PATCH[07-08] support asynchronous page fault. PATCH[09-11] support ioctl and SMCCC interfaces for the functionality. PATHC[12-14] supoort asynchronous page fault for guest PATCH[15] adds document to explain the design and internals Testing ======= The tests are taken using program "testsuite", which is written by myself. The program basically does two things: (a) Starts a thread to allocate all the available memory, write to them by the specified times. (b) The parallel thread is started to do calculation while the memory is written for the specified times. Besides, there are two testing scenarios: (a) the QEMU process is put into cgroup where the memory is limited. With this, we should have asynchronous page fault activities. The total used time for "testsuite" to finish is measured. The calculation capacity is also measured if the corresponding thread is started. (b) To measure the total used time and calculation capacity in scenario of migrating the workload. (a) Running "testsuite" with (cgroup) memory limitation to QEMU process. When the calculation thread isn't started, the consumed time is slightly increased because of overhead introduced by asynchronous page fault. The total used time drops by ~40% when the calculation thread is started. It means the parallelism is greatly improved by the asynchronous page fault. vCPU: 1 Memory: 1024MB cgroupv2.limit: 512MB Command: testsuite test async_pf -l 1 [-t] -q Time- Calculation- Time+ Calculation+ Output -------------------------------------------------------------------- 14.592s 15.010s +2.8% 15.726s 15.185s +3.4% 15.742s 15.192s +3.4% 15.827s 15.270s +3.5% 15.831s 15.291s +3.4% 27.880s 2108m 16.539s 1104m -40.6% -47.6% 27.972s 2111m 16.588s 1110m -40.6% -47.4% 28.020s 2114m 16.656s 1117m -40.5% -47.1% 28.227s 2135m 16.722s 1105m -40.7% -48.2% 28.918s 2194m 16.767s 1113m -42.0% -49.2% Asynchronous page faults: 55000 (b) Migrating the workload ("testsuite"). The total used time is dropped a bit since the migration is completed in very short period (~1.5s). There are not too much asynchronous page fault activies during the migration. It's overall beneficial to migration performance if guest experices high work load. However, the used time is increased by ~14% because of the overhead introduced by asynchronous page fault when guest doesn't have high work load. vCPU: 1 Memory: 1024MB cgroupv2.limit: unlimited Command: testsuite test async_pf -l 50 [-t] -q Time- Calculation- Time+ Calculation+ Output -------------------------------------------------------------------- 11.132s 12.655s +13.6% 11.135s 12.707s +14.1% 11.143s 12.728s +14.2% 11.167s 12.746s +14.1% 11.172s 12.821s +14.7% 27.308s 2252m 25.827s 2131m -5.4% 27.440s 2275m 26.517s 2333m -3.3% 28.069s 2364m 26.520s 2356m -5.5% 28.777s 2427m 26.726s 2383m -7.1% 28.915s 2452m 27.632s 2508m -4.4% migrate.total_time: ~1.6s Asynchronous page faults: ~100 times Changelog ========= v4: * Rebase to v5.14.rc5 and retest (Gavin) v3: * Rebase to v5.13.rc1 (Gavin) * Drop patches from Will to detected SMCCC KVM service (Gavin) * Retest and recapture the benchmarks (Gavin) v2: * Rebase to v5.11.rc6 (Gavin) * Split the patches (James) * Allocate "struct kvm_arch_async_control" dymaicall and use it to check if the feature has been enabled. The kernel option (CONFIG_KVM_ASYNC_PF) isn't used. (James) * Add document to explain the design (James) * Make GFN hash table management generic (James) * Add ioctl commands to support migration (Gavin) Gavin Shan (15): KVM: async_pf: Move struct kvm_async_pf around KVM: async_pf: Add helper function to check completion queue KVM: async_pf: Make GFN slot management generic KVM: x86: Use generic async PF slot management KVM: arm64: Export kvm_handle_user_mem_abort() KVM: arm64: Add paravirtualization header files KVM: arm64: Support page-not-present notification KVM: arm64: Support page-ready notification KVM: arm64: Support async PF hypercalls KVM: arm64: Support async PF ioctl commands KVM: arm64: Export async PF capability arm64: Detect async PF para-virtualization feature arm64: Reschedule process on aync PF arm64: Enable async PF KVM: arm64: Add async PF document Documentation/virt/kvm/arm/apf.rst | 143 +++++++ Documentation/virt/kvm/arm/index.rst | 1 + arch/arm64/Kconfig | 11 + arch/arm64/include/asm/esr.h | 6 + arch/arm64/include/asm/kvm_emulate.h | 27 +- arch/arm64/include/asm/kvm_host.h | 85 ++++ arch/arm64/include/asm/kvm_para.h | 37 ++ arch/arm64/include/asm/processor.h | 1 + arch/arm64/include/asm/thread_info.h | 4 +- arch/arm64/include/uapi/asm/Kbuild | 2 - arch/arm64/include/uapi/asm/kvm.h | 19 + arch/arm64/include/uapi/asm/kvm_para.h | 23 ++ arch/arm64/include/uapi/asm/kvm_sdei.h | 1 + arch/arm64/kernel/Makefile | 1 + arch/arm64/kernel/kvm.c | 452 +++++++++++++++++++++ arch/arm64/kernel/signal.c | 17 + arch/arm64/kvm/Kconfig | 2 + arch/arm64/kvm/Makefile | 1 + arch/arm64/kvm/arm.c | 37 +- arch/arm64/kvm/async_pf.c | 533 +++++++++++++++++++++++++ arch/arm64/kvm/hypercalls.c | 5 + arch/arm64/kvm/mmu.c | 76 +++- arch/arm64/kvm/sdei.c | 5 + arch/x86/include/asm/kvm_host.h | 2 - arch/x86/kvm/Kconfig | 1 + arch/x86/kvm/mmu/mmu.c | 2 +- arch/x86/kvm/x86.c | 88 +--- include/linux/arm-smccc.h | 15 + include/linux/kvm_host.h | 72 +++- include/uapi/linux/kvm.h | 3 + virt/kvm/Kconfig | 3 + virt/kvm/async_pf.c | 95 ++++- virt/kvm/kvm_main.c | 4 +- 33 files changed, 1621 insertions(+), 153 deletions(-) create mode 100644 Documentation/virt/kvm/arm/apf.rst create mode 100644 arch/arm64/include/asm/kvm_para.h create mode 100644 arch/arm64/include/uapi/asm/kvm_para.h create mode 100644 arch/arm64/kernel/kvm.c create mode 100644 arch/arm64/kvm/async_pf.c -- 2.23.0 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
