linux-kernel - Re: [PATCH 1/2] bitmap: add sanity check function for find

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Date:   Tue, 25 Oct 2022 19:26:20 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()

On Tue, Oct 25, 2022 at 10:11:51AM -0700, Yury Norov wrote:
> ARMv6 may or may not support unaligned access depending on SCTLR.U
> bit. This is what Russell was concerned about in the other email.
> As far as I understand, linux enables that feature.

However, we still support ARMv5 and ARMv4, both of which _trap_ every
unaligned access, which will make a findbit call with an unaligned
pointer using word loads painfully expensive. This is the main reason
we haven't used word loads in the findbit ops.

As mentioned, I have patches that do change that (and convert the
thing to use assembly macros to make updates much easier.)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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