netdev - RE: [PATCH net-next 06/10] net/mlx4_core: Fix struct mlx4_vhcr

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Date:	Tue, 27 Jan 2015 09:43:27 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Amir Vadai' <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	"Jack Morgenstein" <jackm@....mellanox.co.il>
Subject: RE: [PATCH net-next 06/10] net/mlx4_core: Fix struct mlx4_vhcr_cmd
 to make implicit padding explicit

From: Amir Vadai
> From: Jack Morgenstein <jackm@....mellanox.co.il>
> 
> Struct mlx4_vhcr was implicitly padded by the gcc compiler.
> This commit makes that padding explicit, to prevent issues with
> changing compilers. Note that we added the padding dword (rather
> than simply packing the structure) in order to maintain compatibility
> with previous kernels.

It isn't a 'compiler' option, but depends on the architecture.

> Reported-by: Alexander Schmidt <alexs@...ux.vnet.ibm.com>
> Signed-off-by: Jack Morgenstein <jackm@....mellanox.co.il>
> Signed-off-by: Amir Vadai <amirv@...lanox.com>
> ---
>  drivers/net/ethernet/mellanox/mlx4/mlx4.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4.h b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> index 096a81c..595e18a 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> @@ -196,13 +196,14 @@ struct mlx4_vhcr {
>  struct mlx4_vhcr_cmd {
>  	__be64 in_param;
>  	__be32 in_modifier;
> +	u32 reserved1;

Adding a pad here changes the alignment on most 32bit architectures (eg i386)
where 64bit integers are only aligned to 4 byte boundaries.
So you are not 'maintaining compatibility with previous kernels'.

>  	__be64 out_param;
>  	__be16 token;
>  	u16 reserved;
>  	u8 status;
>  	u8 flags;
>  	__be16 opcode;
> -};
> +} __packed;

Don't add '__packed' unless you expect the structure to be misaligned
in memory.
On systems that fault mis-aligned memory requests you've requested the
compiler generate code to read/write everything using byte sized memory
accesses and a lot of shifting and masking.

	David

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