linux-kernel - Re: [PATCH net v3] net: marvell: prestera: fix hw structure laid out

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Date:   Fri, 5 Nov 2021 17:56:03 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     "Volodymyr Mytnyk [C]" <vmytnyk@...vell.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Networking <netdev@...r.kernel.org>,
        Taras Chornyi <taras.chornyi@...ision.eu>,
        Mickey Rachamim <mickeyr@...vell.com>,
        Serhiy Pshyk <serhiy.pshyk@...ision.eu>,
        Andrew Lunn <andrew@...n.ch>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Denis Kirjanov <dkirjanov@...e.de>,
        "Taras Chornyi [C]" <tchornyi@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "Vadym Kochan [C]" <vkochan@...vell.com>,
        Yevhen Orlov <yevhen.orlov@...ision.eu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v3] net: marvell: prestera: fix hw structure laid out

On Fri, Nov 5, 2021 at 5:33 PM Volodymyr Mytnyk [C] <vmytnyk@...vell.com> wrote:
> >
> > However, there is still this structure that lacks explicit padding:
> >
> > struct prestera_msg_acl_match {
> >         __le32 type;
> >         /* there is a four-byte hole on most architectures, but not on
> > x86-32 or m68k */
>
> Checked holes on x86_64 with pahole, and there is no holes. Maybe on some
> other there will be. Will add 4byte member here to make sure. Thx.

That is very strange, as the union has an __le64 member that should be
64-bit aligned on x86_64.
> >
> > struct prestera_msg_event_port_param {
> >         union {
> >                 struct {
> >                         __le32 mode;
> >                         __le32 speed;
> >                         u8 oper;
> >                         u8 duplex;
> >                         u8 fc;
> >                         u8 fec;
> >                 } mac;
> >                 struct {
> >                         __le64 lmode_bmap;
> >                         u8 mdix;
> >                         u8 fc;
> >                         u8 __pad[2];
> >                 } __packed phy;
> >         } __packed;
> > } __packed;
> >
> > There is no need to make the outer aggregates __packed, I would
> > mark only the innermost ones here: mode, speed and lmode_bmap.
> > Same for prestera_msg_port_cap_param and prestera_msg_port_param.
> >
>
> Will add __packed only to innermost ones. Looks like only phy is required to have __packed.

I think you need it on both lmode_bmap and mode/speed
to get a completely unaligned structure. If you mark phy as __packed,
that will implicitly mark lmode_bmap as packed but leave the
four-byte alignment on mode and speed, so the entire structure
is still four-byte aligned.

       Arnd