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

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Date:   Mon, 2 Oct 2023 11:49:59 +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 Mon, Oct 02, 2023 at 09:40:04AM +0200, Clément Léger wrote:
> 
> 
> 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).

Ah, I must have misread then. Apologies.

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