linux-kernel - Re: [PATCH v2 05/13] powerpc: use linux/unaligned/le

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Message-ID: <20210514114813.GJ10366@gate.crashing.org>
Date:   Fri, 14 May 2021 06:48:13 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Vineet Gupta <vgupta@...opsys.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 05/13] powerpc: use linux/unaligned/le_struct.h on LE power7

Hi Arnd,

On Fri, May 14, 2021 at 12:00:53PM +0200, Arnd Bergmann wrote:
> Little-endian POWER7 kernels disable
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS because that is not supported on
> the hardware, but the kernel still uses direct load/store for explicti
> get_unaligned()/put_unaligned().
> 
> I assume this is a mistake that leads to power7 having to trap and fix
> up all these unaligned accesses at a noticeable performance cost.
> 
> The fix is completely trivial, just remove the file and use the
> generic version that gets it right.

LE p7 isn't supported (it requires special firmware), and no one uses it
anymore, also not for development.  It was used for powerpc64le-linux
development before p8 was widely available.


Segher