linux-kernel - Re: [PATCH 0/7] Add support to handle misaligned accesses in S-mode

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Message-ID: <20230930-patchy-curdle-ef5ee6e1a17c@spud>
Date:   Sat, 30 Sep 2023 10:23:58 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Clément Léger <cleger@...osinc.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atishp@...osinc.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Evan Green <evan@...osinc.com>,
        Björn Topel <bjorn@...osinc.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ron Minnich <rminnich@...il.com>,
        Daniel Maslowski <cyrevolt@...glemail.com>
Subject: Re: [PATCH 0/7] Add support to handle misaligned accesses in S-mode

On Tue, Sep 26, 2023 at 05:03:09PM +0200, Clément Léger wrote:
> Since commit 61cadb9 ("Provide new description of misaligned load/store
> behavior compatible with privileged architecture.") in the RISC-V ISA
> manual, it is stated that misaligned load/store might not be supported.
> However, the RISC-V kernel uABI describes that misaligned accesses are
> supported. In order to support that, this series adds support for S-mode
> handling of misaligned accesses as well support for prctl(PR_UNALIGN).
> 
> Handling misaligned access in kernel allows for a finer grain control
> of the misaligned accesses behavior, and thanks to the prctl call, can
> allow disabling misaligned access emulation to generate SIGBUS. User
> space can then optimize its software by removing such access based on
> SIGBUS generation.
> 
> Currently, this series is useful for people that uses a SBI that does
> not handled misaligned traps. In a near future, this series will make
> use a SBI extension [1] allowing to request delegation of the
> misaligned load/store traps to the S-mode software. This extension has
> been submitted for review to the riscv tech-prs group. An OpenSBI
> implementation for this spec is available at [2].
> 
> This series can be tested using the spike simulator [3] and an openSBI
> version [4] which allows to always delegate misaligned load/store to
> S-mode.

Some patches in this series do not build for any configs, some are
broken for clang builds and others are broken for nommu. Please try to
build test this more thoroughly before you submit the next version.

Also, AIUI, this series should be marked RFC since the SBI extension
this relies on has not been frozen.

Cheers,
Conor.

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