linux-kernel - Re: [PATCH] mlx5: allow larger xsk chunk

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Date:   Thu, 14 Oct 2021 10:32:36 +0300
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Aya Levin <ayal@...dia.com>,
        Eran Ben Elisha <eranbe@...dia.com>,
        Vladyslav Tarasiuk <vladyslavt@...dia.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] mlx5: allow larger xsk chunk_size



On 10/13/2021 6:02 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> When building with 64KB pages, clang points out that xsk->chunk_size
> can never be PAGE_SIZE:
> 
> drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c:19:22: error: result of comparison of constant 65536 with expression of type 'u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>          if (xsk->chunk_size > PAGE_SIZE ||
>              ~~~~~~~~~~~~~~~ ^ ~~~~~~~~~
> 
> I'm not familiar with the details of this code, but from a quick look
> I found that it gets assigned from a 32-bit variable that can be
> PAGE_SIZE, and that the layout of 'xsk' is not part of an ABI or
> a hardware structure, so extending the members to 32 bits as well
> should address both the behavior on 64KB page kernels, and the
> warning I saw.
> 
> In older versions of this code, using PAGE_SIZE was the only
> possibility, so this would have never worked on 64KB page kernels,
> but the patch apparently did not address this case completely.
> 
> Fixes: 282c0c798f8e ("net/mlx5e: Allow XSK frames smaller than a page")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en/params.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/params.h b/drivers/net/ethernet/mellanox/mlx5/core/en/params.h
> index 879ad46d754e..b4167350b6df 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/params.h
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/params.h
> @@ -7,8 +7,8 @@
>   #include "en.h"
>   
>   struct mlx5e_xsk_param {
> -	u16 headroom;
> -	u16 chunk_size;
> +	u32 headroom;
> +	u32 chunk_size;

Hi Arnd,

I agree with your arguments about chunk_size.
Yet I have mixed feelings about extending the headroom. Predating 
in-driver code uses u16 for headroom (i.e. [1]), while 
xsk_pool_get_headroom returns u32.

[1] drivers/net/ethernet/mellanox/mlx5/core/en/params.c :: 
mlx5e_get_linear_rq_headroom

As this patch is a fix, let's keep it minimal, only addressing the issue 
described in title and description.
We might want to move headroom to u32 all around the driver in a 
separate patch to -next.

>   };
>   
>   struct mlx5e_lro_param {
>