linux-kernel - Re: [PATCH] ARM: unaligned.h: Use an arch-specific version

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Message-ID: <20170920154143.GO20805@n2100.armlinux.org.uk>
Date:   Wed, 20 Sep 2017 16:41:43 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Romain Izard <romain.izard.pro@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] ARM: unaligned.h: Use an arch-specific version

On Wed, Sep 20, 2017 at 08:26:09AM -0700, Ard Biesheuvel wrote:
> Hi Romain,
> 
> On 20 September 2017 at 08:18, Romain Izard <romain.izard.pro@...il.com> wrote:
> > For the 32-bit ARM architecture, unaligned access support is variable.
> > On a chip without a MMU, an unaligned access returns a rotated data word
> > and must be avoided.
> >
> > When a MMU is available, it can be trapped. On ARMv6 or ARMv7, it can also
> > be handled by the hardware, depending on the type of access instruction.
> > Unaligned access of 32 bits or less are corrected, while larger access
> > provoke a trap.
> >
> > Unfortunately, the compiler is able to merge two 32-bit access that
> > would generate a LDR instruction, that works on unaligned access, into a
> > single LDM access that will not work. This is not a common situation,
> > but it has been observed in the LZ4 decompression code.
> >
> > To prevent this type of optimization, it is necessary to change the
> > explicit accessors for unaligned addresses from those defined in the
> > access_ok.h header, to those defined in the packed_struct.h header.
> >
> > Add an arch-specific header to ARM, to retain other optimizations that
> > rely on HAVE_EFFICIENT_UNALIGNED_ACCESS, while making sure that access
> > that explicitly rely on the unaligned accessors are correctly handled by
> > the compiler.
> >
> > Signed-off-by: Romain Izard <romain.izard.pro@...il.com>
> > ---
> >
> 
> If access_ok.h has been observed to produce different output from the
> struct versions (using any compiler), I guess we cannot simply change
> the asm-generic default and expect everybody to be ok with that. So I
> agree this is the most appropriate course of action.

However, what effect does this have on the code generated for the rest
of the kernel?

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