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  1. Sep 02, 2023
    • Evan Green's avatar
      RISC-V: Probe for unaligned access speed · 584ea656
      Evan Green authored
      
      
      Rather than deferring unaligned access speed determinations to a vendor
      function, let's probe them and find out how fast they are. If we
      determine that an unaligned word access is faster than N byte accesses,
      mark the hardware's unaligned access as "fast". Otherwise, we mark
      accesses as slow.
      
      The algorithm itself runs for a fixed amount of jiffies. Within each
      iteration it attempts to time a single loop, and then keeps only the best
      (fastest) loop it saw. This algorithm was found to have lower variance from
      run to run than my first attempt, which counted the total number of
      iterations that could be done in that fixed amount of jiffies. By taking
      only the best iteration in the loop, assuming at least one loop wasn't
      perturbed by an interrupt, we eliminate the effects of interrupts and
      other "warm up" factors like branch prediction. The only downside is it
      depends on having an rdtime granular and accurate enough to measure a
      single copy. If we ever manage to complete a loop in 0 rdtime ticks, we
      leave the unaligned setting at UNKNOWN.
      
      There is a slight change in user-visible behavior here. Previously, all
      boards except the THead C906 reported misaligned access speed of
      UNKNOWN. C906 reported FAST. With this change, since we're now measuring
      misaligned access speed on each hart, all RISC-V systems will have this
      key set as either FAST or SLOW.
      
      Currently, we don't have a way to confidently measure the difference between
      SLOW and EMULATED, so we label anything not fast as SLOW. This will
      mislabel some systems that are actually EMULATED as SLOW. When we get
      support for delegating misaligned access traps to the kernel (as opposed
      to the firmware quietly handling it), we can explicitly test in Linux to
      see if unaligned accesses trap. Those systems will start to report
      EMULATED, though older (today's) systems without that new SBI mechanism
      will continue to report SLOW.
      
      I've updated the documentation for those hwprobe values to reflect
      this, specifically: SLOW may or may not be emulated by software, and FAST
      represents means being faster than equivalent byte accesses. The change
      in documentation is accurate with respect to both the former and current
      behavior.
      
      Signed-off-by: default avatarEvan Green <evan@rivosinc.com>
      Acked-by: default avatarConor Dooley <conor.dooley@microchip.com>
      Link: https://lore.kernel.org/r/20230818194136.4084400-2-evan@rivosinc.com
      
      
      Signed-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
      584ea656
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