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

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Date:   Wed, 4 Oct 2023 08:26:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Clément Léger' <cleger@...osinc.com>,
        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-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.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

From: Clément Léger
> Sent: 02 October 2023 08:40
> 
> 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.
...

That it just really horrid.
If the cpu is going to trap misaligned accesses then you want
The compiler generated code (ie packed data) not to generate
misaligned accesses.
So you have to change the kernel uABI.

OTOH if you known that such accesses won't fault and will be
not really slower than aligned accesses then optimised versions
of some functions (like memcpy and checksums) can use misaligned
accesses.

	David

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