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authorEvan Green <evan@rivosinc.com>2023-08-18 12:41:35 -0700
committerPalmer Dabbelt <palmer@rivosinc.com>2023-09-01 09:06:25 -0700
commit584ea6564bcaead223840cdafe84e4947ba85509 (patch)
tree8256f053ae45a3c9cdd79e0d8f01900714bb5050 /Documentation
parent06c2afb862f9da8dc5efa4b6076a0e48c3fbaaa5 (diff)
RISC-V: Probe for unaligned access speed
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: Evan Green <evan@rivosinc.com> Acked-by: Conor Dooley <conor.dooley@microchip.com> Link: https://lore.kernel.org/r/20230818194136.4084400-2-evan@rivosinc.com Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/riscv/hwprobe.rst11
1 files changed, 5 insertions, 6 deletions
diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
index 19165ebd82ba..f63fd05f1a73 100644
--- a/Documentation/riscv/hwprobe.rst
+++ b/Documentation/riscv/hwprobe.rst
@@ -87,13 +87,12 @@ The following keys are defined:
emulated via software, either in or below the kernel. These accesses are
always extremely slow.
- * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned accesses are supported
- in hardware, but are slower than the cooresponding aligned accesses
- sequences.
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_SLOW`: Misaligned accesses are slower
+ than equivalent byte accesses. Misaligned accesses may be supported
+ directly in hardware, or trapped and emulated by software.
- * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned accesses are supported
- in hardware and are faster than the cooresponding aligned accesses
- sequences.
+ * :c:macro:`RISCV_HWPROBE_MISALIGNED_FAST`: Misaligned accesses are faster
+ than equivalent byte accesses.
* :c:macro:`RISCV_HWPROBE_MISALIGNED_UNSUPPORTED`: Misaligned accesses are
not supported at all and will generate a misaligned address fault.