linux-kernel - Re: [PATCH v2] Fixed: Misaligned memory access. Fixed pointer comparison.

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Message-Id: <485047B2-E566-4679-87CF-C4B3CAFEF108@jrtc27.com>
Date:   Sun, 23 Jan 2022 15:44:51 +0000
From:   Jessica Clarke <jrtc27@...c27.com>
To:     "Michael T. Kloos" <michael@...haelkloos.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Fixed: Misaligned memory access. Fixed pointer
 comparison.

On 23 Jan 2022, at 03:45, Michael T. Kloos <michael@...haelkloos.com> wrote:
> 
> Rewrote the riscv memmove() assembly implementation.  The
> previous implementation did not check memory alignment and it
> compared 2 pointers with a signed comparison.  The misaligned
> memory access would cause the kernel to crash on systems that
> did not emulate it in firmware and did not support it in hardware.
> Firmware emulation is slow and may not exist.  Additionally,
> hardware support may not exist and would likely still run slower
> than aligned accesses even if it did.  The RISC-V spec does not
> guarantee that support for misaligned memory accesses will exist.
> It should not be depended on.
> 
> This patch now checks for the maximum granularity of co-alignment
> between the pointers and copies them with that, using single-byte
> copy for any unaligned data at their terminations.  It also now uses
> unsigned comparison for the pointers.
> 
> Added half-word and, if built for 64-bit, double-word copy.
> 
> Migrated to the	newer assembler annotations from the now deprecated
> ones.
> 
> Commit Message Edited on Jan 22 2022: Fixed some typos.
> 
> [v2]
> 
> Per kernel test robot, I have fixed the build under clang.  This
> was broken due to a difference between gcc and clang, clang requiring
> explict zero offsets the jalr instruction. gcc allowed them to be
> omitted if zero.

Unlike LLVM, GCC does not have an assembler, that’s binutils’s GNU as.

Jess