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

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Message-ID: <8ce6cd97-6d63-4174-a290-40690c81e205@rivosinc.com>
Date:   Mon, 2 Oct 2023 09:40:04 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Conor Dooley <conor@...nel.org>
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 30/09/2023 11:23, Conor Dooley wrote:
> 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.

Hi Conor,

Thanks for the feedback, I'll check that.

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

This series does not actually uses the SBI extension but provides a way
to detect if misaligned accesses are not handled by hardware nor by the
SBI. It has been reported by Ron & Daniel they they have a minimal SBI
implementation that does not handle misaligned accesses and that they
would like to make use of the PR_SET_UNALIGN feature. This is what this
series addresses (and thus does not depend on the mentioned SBI extension).

Thanks,

Clément

> 
> Cheers,
> Conor.