netdev - Re: [PATCH 08/20] octeontx2-af: Alloc and config NPC MCAM entry at a time

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Date:   Fri, 9 Nov 2018 12:02:41 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Sunil Kovvuri <sunil.kovvuri@...il.com>
Cc:     Networking <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, linux-soc@...r.kernel.org,
        Sunil Goutham <sgoutham@...vell.com>
Subject: Re: [PATCH 08/20] octeontx2-af: Alloc and config NPC MCAM entry at a time

On Fri, Nov 9, 2018 at 5:21 AM Sunil Kovvuri <sunil.kovvuri@...il.com> wrote:
>
> On Fri, Nov 9, 2018 at 2:13 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Thu, Nov 8, 2018 at 7:37 PM <sunil.kovvuri@...il.com> wrote:
> > > @@ -666,4 +668,20 @@ struct npc_mcam_unmap_counter_req {
> > >         u8  all;   /* Unmap all entries using this counter ? */
> > >  };
> > >
> > > +struct npc_mcam_alloc_and_write_entry_req {
> > > +       struct mbox_msghdr hdr;
> > > +       struct mcam_entry entry_data;
> > > +       u16 ref_entry;
> > > +       u8  priority;    /* Lower or higher w.r.t ref_entry */
> > > +       u8  intf;        /* Rx or Tx interface */
> > > +       u8  enable_entry;/* Enable this MCAM entry ? */
> > > +       u8  alloc_cntr;  /* Allocate counter and map ? */
> > > +};
> >
> > I noticed that this structure requires padding at the end because
> > struct mbox_msghdr has a 32-bit alignment requirement. For
> > data structures in an interface, I'd recommend avoiding that kind
> > of padding and adding reserved fields or widening the types
> > accordingly.
> >
>
> When there are multiple messages in the mailbox, each message starts
> at a 16byte aligned offset. So struct mbox_msghdr is always aligned.
> I think adding reserved fields is not needed here.
>
> ===
> struct mbox_msghdr *otx2_mbox_alloc_msg_rsp(struct otx2_mbox *mbox, int devid,
>                                             int size, int size_rsp)
> {
>         size = ALIGN(size, MBOX_MSG_ALIGN);
> ===
>
> Is this what you were referring to ?
>

No, I mean the padding at the end of the structure. An example
would be a structure like

struct s {
    u16 a;
    u32 b;
    u16 c;
};

Since b is aligned to four bytes, you get padding between a and b.
On top of that, you also get padding after c to make the size of
structure itself be a multiple of its alignment. For interfaces, we
should avoid both kinds of padding. This can be done by marking
members as __packed (usually I don't recommend that), by
changing the size of members, or by adding explicit 'reserved'
fields in place of the padding.

> > I also noticed a similar problem in struct mbox_msghdr. Maybe
> > use the 'pahole' tool to check for this kind of padding in the
> > API structures.

     Arnd