linux-kernel - Re: [PATCH 2/2] riscv: uaccess: do not do misaligned accesses in get/put

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Message-ID: <alpine.DEB.2.21.2506011830130.28395@angie.orcam.me.uk>
Date: Sun, 1 Jun 2025 18:35:13 +0100 (BST)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: David Laight <david.laight.linux@...il.com>
cc: Clément Léger <cleger@...osinc.com>, 
    linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
    Paul Walmsley <paul.walmsley@...ive.com>, 
    Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
    Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH 2/2] riscv: uaccess: do not do misaligned accesses in
 get/put_user()

On Sat, 31 May 2025, David Laight wrote:

> > Such behavior was detected with GET_UNALIGN_CTL() that was doing
> > a put_user() with an unsigned long* address that should have been an
> > unsigned int*. Reenabling kernel misaligned access emulation is a bit
> > risky and it would also degrade performances. Rather than doing that,
> > we will try to avoid any misaligned accessed by using copy_from/to_user()
> > which does not do any misaligned accesses. This can be done only for
> > !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus allows to only generate
> > a bit more code for this config.
> 
> For get_user() you are much better off reading the two words that contain
> the value and then doing 'shift' and 'or' to get the correct value.
> 
> Even for put_user() doing the explicit byte accesses will be faster than
> going though the generic copy_to/from_user() function.

 FWIW I think optimising copy_to/from_user for such cases would be a more 
robust approach moving forward than sprinkling open-coded implementations 
across code.

  Maciej