Hi, In this series we introduce Event-Based Branch (EBB) support for PPC64 TCG. EBB consists of a new instruction called 'rfebb' and interrupt/exception logic that redirects the userspace next intruction pointer to another address, and the rfebb instruction restores execution back afterwards.
One way to exercise EBB is the Linux kernel selftests, located in the kernel tree tools/testing/selftests/powerpc/pmu/ebb). These tests uses the PMU (Performance Monitor Unit). We don't have a PPC64 PMU implementation available, so to validate the EBB implementation we also introduced a rudimentary PPC64 PMU for TCG. The PMU will not count anything but instructions and cycles. There is no support for sampled/random events that are enabled by MMCR2. Not all MMCR0 bits are being used. The limitations and capabilities of this PMU implementation is described in the docs in the last patch. This work was started by Gustavo Romero in 2020. You can find Gustavo's patches in his branch at [1]. Gustavo's current work email is CCed in the patches so he can follow and participate in the review process if he wants to. These patches can be divided in 3 parts: - Patches 1 to 8: basic PMU support for instructions/cycles counting - Patches 9 to 11: EBB implementation - Patches 12 to 18: implement counter negative PMU alerts to trigger EBB exceptions Both the PMU and EBB are based on the PowerISA 3.1 specification. One notable limitation of the PMU implementation is the icount precision. Using the PMU powerpc kernel tests, located in tools/testing/selftests/powerpc/pmu, we will consistently fall short in tests where 10M or more instructions are sampled, with an error rate of around 0,07%. I am not certain whether this has to do with the logic implemented here or with an icount precision limit, or both. Since our objective here is to validate the EBB logic I am overlooking these errors. All that said, any input/feedback related to how we can improve the instruction count is welcome. The benchmark for this implementation is the EBB powerpc kernel tests, located in the kernel tree in tools/testing/selftests/powerpc/pmu/ebb. Of the current 22 EBB tests presented in the kernel v5.13 we can pass 20 of those all the time. . 'instructions_count_test' suffers from the same limitations of the PMU tests mentioned above. 'lost_exception_test' will pass sometimes, although the most common scenario is a failure. Since this is a heavy focused PMU test that happens to use EBB I am also overlooking its failure. [1] https://github.com/gromero/qemu/commits/ebb Daniel Henrique Barboza (15): target/ppc: add exclusive Book3S PMU reg read/write functions target/ppc: PMU Book3s basic insns count for pseries TCG target/ppc/pmu_book3s_helper.c: eliminate code repetition target/ppc/pmu_book3s_helper: enable PMC1-PMC4 events target/ppc/pmu_book3s_helper.c: icount fine tuning target/ppc/pmu_book3s_helper.c: do an actual cycles calculation target/ppc/excp_helper.c: POWERPC_EXCP_EBB adjustments target/ppc/pmu_book3s_helper.c: enable PMC1 counter negative EBB target/ppc/translate: PMU: handle setting of PMCs while running target/ppc/pmu_book3s_helper.c: add generic timeout helpers target/ppc/pmu_book3s_helper: enable counter negative for all PMCs target/ppc/pmu_book3s_helper: adding 0xFA event target/ppc/pmu_book3s_helper.c: add PMC14/PMC56 counter freeze bits target/ppc/pmu_book3s_helper.c: add PM_CMPLU_STALL mock events docs/specs: add PPC64 TCG PMU-EBB documentation Gustavo Romero (4): target/ppc: add exclusive user read function for PMU regs target/ppc: add exclusive user write function for PMU regs PPC64/TCG: Implement 'rfebb' instruction target/ppc: PMU Event-Based exception support docs/specs/index.rst | 1 + docs/specs/ppc-tcg-pmu-ebb.rst | 71 +++++ hw/ppc/spapr_cpu_core.c | 6 + target/ppc/cpu.h | 47 +++- target/ppc/cpu_init.c | 54 ++-- target/ppc/excp_helper.c | 61 +++++ target/ppc/helper.h | 2 + target/ppc/meson.build | 1 + target/ppc/pmu_book3s_helper.c | 466 +++++++++++++++++++++++++++++++++ target/ppc/spr_tcg.h | 4 + target/ppc/translate.c | 151 ++++++++++- 11 files changed, 834 insertions(+), 30 deletions(-) create mode 100644 docs/specs/ppc-tcg-pmu-ebb.rst create mode 100644 target/ppc/pmu_book3s_helper.c -- 2.31.1
