linux-kernel - Re: [PATCH net-next v2] net: ncsi: Fix GCPS 64-bit member variables

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Message-ID: <Z/eiki2mlBiAeBrc@home.paul.comp>
Date: Thu, 10 Apr 2025 13:50:58 +0300
From: Paul Fertser <fercerpav@...il.com>
To: kalavakunta.hari.prasad@...il.com
Cc: sam@...dozajonas.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, horms@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        npeacock@...a.com, akozlov@...a.com, hkalavakunta@...a.com
Subject: Re: [PATCH net-next v2] net: ncsi: Fix GCPS 64-bit member variables

Hello Hari,

Thank you for the patch, it looks really clean. However I have one
more question now.

On Wed, Apr 09, 2025 at 06:23:08PM -0700, kalavakunta.hari.prasad@...il.com wrote:
> @@ -290,11 +289,11 @@ struct ncsi_rsp_gcps_pkt {
>  	__be32                  tx_1023_frames; /* Tx 512-1023 bytes frames   */
>  	__be32                  tx_1522_frames; /* Tx 1024-1522 bytes frames  */
>  	__be32                  tx_9022_frames; /* Tx 1523-9022 bytes frames  */
> -	__be32                  rx_valid_bytes; /* Rx valid bytes             */
> +	__be64                  rx_valid_bytes; /* Rx valid bytes             */
>  	__be32                  rx_runt_pkts;   /* Rx error runt packets      */
>  	__be32                  rx_jabber_pkts; /* Rx error jabber packets    */
>  	__be32                  checksum;       /* Checksum                   */
> -};
> +}  __packed __aligned(4);

This made me check the Specification and indeed somehow it happened
that they have forgotten to ensure natural alignment for 64-bit fields
(at least they cared enough to do it for 32-bit values). [0] is the
relevant read.

> +	ncs->hnc_cnt            = be64_to_cpu(rsp->cnt);

This means that while it works fine on common BMCs now (since they run
in 32-bit mode) the access will be trappped as unaligned on 64-bit
Arms which one day will be common (Aspeed AST2700, Nuvoton NPCM8XX).

So I guess you should be doing `be64_to_cpup(&rsp->cnt)` there.

[0] https://www.catb.org/esr/structure-packing/