The current setting for the hwprobe bit indicating misaligned access speed is controlled by a vendor-specific feature probe function. This is essentially a per-SoC table we have to maintain on behalf of each vendor going forward. Let's convert that instead to something we detect at runtime. We have two assembly routines at the heart of our probe: one that does a bunch of word-sized accesses (without aligning its input buffer), and the other that does byte accesses. If we can move a larger number of bytes using misaligned word accesses than we can with the same amount of time doing byte accesses, then we can declare misaligned accesses as "fast". The tradeoff of reducing this maintenance burden is boot time. We spend 4-6 jiffies per core doing this measurement (0-2 on jiffie edge alignment, and 4 on measurement). The timing loop was based on raid6_choose_gen(), which uses (16+1)*N jiffies (where N is the number of algorithms). By taking only the fastest iteration out of all attempts for use in the comparison, variance between runs is very low. On my THead C906, it looks like this: [ 0.047563] cpu0: Ratio of byte access time to unaligned word access is 4.34, unaligned accesses are fast Several others have chimed in with results on slow machines with the older algorithm, which took all runs into account, including noise like interrupts. Even with this variation, results indicate that in all cases (fast, slow, and emulated) the measured numbers are nowhere near each other (always multiple factors away). Changes in v4: - Avoid the bare 64-bit divide which fails to link on 32-bit systems, use div_u64() (Palmer, buildrobot) Changes in v3: - Fix documentation indentation (Conor) - Rename __copy_..._unaligned() to __riscv_copy_..._unaligned() (Conor) - Renamed c0,c1 to start_cycles, end_cycles (Conor) - Renamed j0,j1 to start_jiffies, now - Renamed check_unaligned_access0() to check_unaligned_access_boot_cpu() (Conor) Changes in v2: - Explain more in the commit message (Conor) - Use a new algorithm that looks for the fastest run (David) - Clarify documentatin further (David and Conor) - Unify around a single word, "unaligned" (Conor) - Align asm operands, and other misc whitespace changes (Conor) Evan Green (2): RISC-V: Probe for unaligned access speed RISC-V: alternative: Remove feature_probe_func Documentation/riscv/hwprobe.rst | 11 ++- arch/riscv/errata/thead/errata.c | 8 --- arch/riscv/include/asm/alternative.h | 5 -- arch/riscv/include/asm/cpufeature.h | 2 + arch/riscv/kernel/Makefile | 1 + arch/riscv/kernel/alternative.c | 19 ----- arch/riscv/kernel/copy-unaligned.S | 71 ++++++++++++++++++ arch/riscv/kernel/copy-unaligned.h | 13 ++++ arch/riscv/kernel/cpufeature.c | 104 +++++++++++++++++++++++++++ arch/riscv/kernel/smpboot.c | 3 +- 10 files changed, 198 insertions(+), 39 deletions(-) create mode 100644 arch/riscv/kernel/copy-unaligned.S create mode 100644 arch/riscv/kernel/copy-unaligned.h -- 2.34.1
