* [PATCH] doc: RISC-V: Document that misaligned accesses are supported
@ 2022-07-28 21:07 Palmer Dabbelt
2022-10-12 15:59 ` Palmer Dabbelt
0 siblings, 1 reply; 2+ messages in thread
From: Palmer Dabbelt @ 2022-07-28 21:07 UTC (permalink / raw)
To: corbet
Cc: Paul Walmsley, Palmer Dabbelt, aou, bagasdotme, linux-doc,
linux-riscv, linux-kernel, linux, Palmer Dabbelt
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
_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] doc: RISC-V: Document that misaligned accesses are supported
2022-07-28 21:07 [PATCH] doc: RISC-V: Document that misaligned accesses are supported Palmer Dabbelt
@ 2022-10-12 15:59 ` Palmer Dabbelt
0 siblings, 0 replies; 2+ messages in thread
From: Palmer Dabbelt @ 2022-10-12 15:59 UTC (permalink / raw)
To: Palmer Dabbelt, corbet
Cc: linux, Paul Walmsley, linux-doc, bagasdotme, linux-kernel, aou,
Palmer Dabbelt, linux-riscv
On Thu, 28 Jul 2022 14:07:15 -0700, Palmer Dabbelt wrote:
> 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.
>
> [...]
Applied, thanks!
[1/1] doc: RISC-V: Document that misaligned accesses are supported
commit: 06267eb2decaa6baac81bbd882265a8e7782dba4
Best regards,
--
Palmer Dabbelt <palmer@rivosinc.com>
_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv
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