netdev - Re: [PATCH net-next 1/3] sfc: use padding to fix alignment in loopback test

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Date: Thu, 22 Jun 2023 13:38:54 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: edward.cree@....com, linux-net-drivers@....com,
 "David S . Miller" <davem@...emloft.net>, "Jakub Kicinski" <kuba@...nel.org>,
 "Eric Dumazet" <edumazet@...gle.com>, "Paolo Abeni" <pabeni@...hat.com>
Cc: "Edward Cree" <ecree.xilinx@...il.com>, Netdev <netdev@...r.kernel.org>,
 "Martin Habets" <habetsm.xilinx@...il.com>
Subject: Re: [PATCH net-next 1/3] sfc: use padding to fix alignment in loopback test

On Thu, Jun 22, 2023, at 12:18, edward.cree@....com wrote:
>  drivers/net/ethernet/sfc/selftest.c | 45 +++++++++++++++++------------
>  1 file changed, 27 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/net/ethernet/sfc/selftest.c 
> b/drivers/net/ethernet/sfc/selftest.c
> index 3c5227afd497..74b42efe7267 100644
> --- a/drivers/net/ethernet/sfc/selftest.c
> +++ b/drivers/net/ethernet/sfc/selftest.c
> @@ -42,12 +42,16 @@
>   * Falcon only performs RSS on TCP/UDP packets.
>   */
>  struct efx_loopback_payload {
> +	char pad[2]; /* Ensures ip is 4-byte aligned */
>  	struct ethhdr header;
>  	struct iphdr ip;
>  	struct udphdr udp;
>  	__be16 iteration;
>  	char msg[64];
>  } __packed;

There should probably be an extra __aligned(4) after the __packed,
so that the compiler knows the start of the structure is aligned,
otherwise (depending on the architecture and compiler), any access
to a member can still turn into multiple single-byte accesses.

I would also expect to still see a warning without that extra
attribute. Aside from this, the patch looks good to me.

     Arnd