[PATCH] arm64: mm: define NET_IP_ALIGN to 0

[will.deacon@arm.com (Will Deacon)]

On Wed, Nov 07, 2018 at 06:10:38PM +0100, Ard Biesheuvel wrote:
> On arm64, there is no need to add 2 bytes of padding to the start of
> each network buffer just to make the IP header appear 32-bit aligned.
> 
> Since this might actually adversely affect DMA performance some
> platforms, let's override NET_IP_ALIGN to 0 to get rid of this
> padding.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm64/include/asm/processor.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 3e2091708b8e..6b0d4dff5012 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -24,6 +24,14 @@
>  #define KERNEL_DS	UL(-1)
>  #define USER_DS		(TASK_SIZE_64 - 1)
>  
> +/*
> + * On arm64 systems, unaligned accesses by the CPU are cheap, and so there is
> + * no point in shifting all network buffers by 2 bytes just to make some IP
> + * header fields appear aligned in memory, potentially sacrificing some DMA
> + * performance on some platforms.
> + */
> +#define NET_IP_ALIGN	0

Acked-by: Will Deacon <will.deacon@arm.com>

If you end up refactoring this in the core code as Mark suggested, that's
great, but I don't think it should hold this patch up.

Will