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Subject: [PATCH] doc: RISC-V: Document that misaligned accesses are supported
Date:   Thu, 28 Jul 2022 14:07:15 -0700
Message-Id: <20220728210715.17214-1-palmer@rivosinc.com>
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Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>, aou@eecs.berkeley.edu,
        bagasdotme@gmail.com, linux-doc@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux@rivosinc.com, Palmer Dabbelt <palmer@rivosinc.com>
From:   Palmer Dabbelt <palmer@rivosinc.com>
To:     corbet@lwn.net
Precedence: bulk

From: Palmer Dabbelt <palmer@rivosinc.com>

The RISC-V ISA manual used to mandate that misaligned accesses were
supported in user mode, but that requirement was removed in 2018 via
riscv-isa-manual commit 61cadb9 ("Provide new description of misaligned
load/store behavior compatible with privileged architecture.").  Since
the Linux uABI was already frozen at that point it's just been demoted
to part of the uABI, but that was never written down.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

---

We currently rely on either hardware or M-mode to support misaligned
accesses.  It's probably worth supporting the PR_SET_UNALIGN so
userspace can opt out of the emulation routines, but we can't break the
uABI so we're stuck with defaulting to them on for now.  Doing so would
also require some way of informing M-mode that unsupported accesses
shouldn't be emulated.
---
 Documentation/riscv/index.rst | 1 +
 Documentation/riscv/uabi.rst  | 6 ++++++
 2 files changed, 7 insertions(+)
 create mode 100644 Documentation/riscv/uabi.rst

diff --git a/Documentation/riscv/index.rst b/Documentation/riscv/index.rst
index e23b876ad6eb..2e5b18fbb145 100644
--- a/Documentation/riscv/index.rst
+++ b/Documentation/riscv/index.rst
@@ -8,6 +8,7 @@ RISC-V architecture
     boot-image-header
     vm-layout
     patch-acceptance
+    uabi
 
     features
 
diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst
new file mode 100644
index 000000000000..21a82cfb6c4d
--- /dev/null
+++ b/Documentation/riscv/uabi.rst
@@ -0,0 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+RISC-V Linux User ABI
+=====================
+
+Misaligned accesses are supported in userspace, but they may perform poorly.
-- 
2.34.1