Documentation: RISC-V: uabi: Only scalar misaligned loads are supported

We're stuck supporting scalar misaligned loads in userspace because they
were part of the ISA at the time we froze the uABI.  That wasn't the
case for vector misaligned accesses, so depending on them
unconditionally is a userspace bug.  All extant vector hardware traps on
these misaligned accesses.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240524185600.5919-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/Documentation/arch/riscv/uabi.rst b/Documentation/arch/riscv/uabi.rst
index 54d199d..2b420ba 100644 (file)
--- a/Documentation/arch/riscv/uabi.rst
+++ b/Documentation/arch/riscv/uabi.rst
@@ -65,4 +65,6 @@ the extension, or may have deliberately removed it from the listing.
 Misaligned accesses
 -------------------
 
-Misaligned accesses are supported in userspace, but they may perform poorly.
+Misaligned scalar accesses are supported in userspace, but they may perform
+poorly.  Misaligned vector accesses are only supported if the Zicclsm extension
+is supported.