Re: [PATCH v3 2/2] net: have checksum routines accept unaligned data

[Olivier Matz <olivier.matz@6wind.com>]

On Mon, Jul 11, 2022 at 02:11:32PM +0200, Mattias Rönnblom wrote:
> __rte_raw_cksum() (used by rte_raw_cksum() among others) accessed its
> data through an uint16_t pointer, which allowed the compiler to assume
> the data was 16-bit aligned. This in turn would, with certain
> architectures and compiler flag combinations, result in code with SIMD
> load or store instructions with restrictions on data alignment.
> 
> This patch keeps the old algorithm, but data is read using memcpy()
> instead of direct pointer access, forcing the compiler to always
> generate code that handles unaligned input. The __may_alias__ GCC
> attribute is no longer needed.
> 
> The data on which the Internet checksum functions operates are almost
> always 16-bit aligned, but there are exceptions. In particular, the
> PDCP protocol header may (literally) have an odd size.
> 
> Performance impact seems to range from none to a very slight
> regression.
> 
> Bugzilla ID: 1035
> Cc: stable@dpdk.org

Fixes: 6006818cfb26 ("net: new checksum functions")

> ---
> 
> v3:
>   * Use RTE_ALIGN_FLOOR() in the pointer arithmetic (Olivier Matz).
> v2:
>   * Simplified the odd-length conditional (Morten Brørup).
> 
> Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
> 
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>

Acked-by: Olivier Matz <olivier.matz@6wind.com>

Thank you!