[PATCH v2 0/2] RISC-V: Probe for misaligned access speed

[Evan Green]

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 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