linux-kernel - RE: [net PATCH] octeontx2-af: Fix marking couple of structure as _

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Date: Tue, 19 Dec 2023 15:26:11 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Jacob Keller' <jacob.e.keller@...el.com>, Suman Ghosh
	<sumang@...vell.com>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"sgoutham@...vell.com" <sgoutham@...vell.com>, "sbhatta@...vell.com"
	<sbhatta@...vell.com>, "jerinj@...vell.com" <jerinj@...vell.com>,
	"gakula@...vell.com" <gakula@...vell.com>, "hkelam@...vell.com"
	<hkelam@...vell.com>, "lcherian@...vell.com" <lcherian@...vell.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [net PATCH] octeontx2-af: Fix marking couple of structure as
 __packed

From: Jacob Keller
> Sent: 18 December 2023 20:44
> 
> On 12/18/2023 12:27 AM, Suman Ghosh wrote:
> > Couple of structures was not marked as __packed which may have some
> > performance implication. This patch fixes the same and mark them as
> > __packed.
> 
> Not sure I follow why lack of __packed would have performance
> implications? I get that __packed is important to ensure layout is
> correct or to ensure the whole structure has the right size rather than
> unexpected gaps. I'd guess maybe because the structures size would
> include padding without __packed, leading to a lot of gaps when
> combining several structures together...
> 
> I did test on my system with pahole, and even without __packed, I don't
> get any gaps in the npc_lt_def_cfg structure:
> 
> 
> > struct npc_lt_def_cfg {
> >         struct npc_lt_def          rx_ol2;               /*     0     3 */
> >         struct npc_lt_def          rx_oip4;              /*     3     3 */
> >         struct npc_lt_def          rx_iip4;              /*     6     3 */
> >         struct npc_lt_def          rx_oip6;              /*     9     3 */
> >         struct npc_lt_def          rx_iip6;              /*    12     3 */
> >         struct npc_lt_def          rx_otcp;              /*    15     3 */
> >         struct npc_lt_def          rx_itcp;              /*    18     3 */
> >         struct npc_lt_def          rx_oudp;              /*    21     3 */
> >         struct npc_lt_def          rx_iudp;              /*    24     3 */
> >         struct npc_lt_def          rx_osctp;             /*    27     3 */
> >         struct npc_lt_def          rx_isctp;             /*    30     3 */
> >         struct npc_lt_def_ipsec    rx_ipsec[2];          /*    33    10 */
> >         struct npc_lt_def          pck_ol2;              /*    43     3 */
> >         struct npc_lt_def          pck_oip4;             /*    46     3 */
> >         struct npc_lt_def          pck_oip6;             /*    49     3 */
> >         struct npc_lt_def          pck_iip4;             /*    52     3 */
> >         struct npc_lt_def_apad     rx_apad0;             /*    55     4 */
> >         struct npc_lt_def_apad     rx_apad1;             /*    59     4 */
> >         struct npc_lt_def_color    ovlan;                /*    63     5 */
> >         /* --- cacheline 1 boundary (64 bytes) was 4 bytes ago --- */
> >         struct npc_lt_def_color    ivlan;                /*    68     5 */
> >         struct npc_lt_def_color    rx_gen0_color;        /*    73     5 */
> >         struct npc_lt_def_color    rx_gen1_color;        /*    78     5 */
> >         struct npc_lt_def_et       rx_et[2];             /*    83    10 */
> >
> >         /* size: 93, cachelines: 2, members: 23 */
> >         /* last cacheline: 29 bytes */
> > };
> 
> 
> However that may not be true across all compilers etc. Also all the
> other structures are __packed. Makes sense.

Or not - maybe all the __packed should be removed instead!

Unless these structures (or any others) appear in 'messages' which
get transferred between systems they really shouldn't be __packed.
And a 93 byte 'message' with all those fields seems rather odd.

The above breakdown seems to imply everything is 'unsigned char'
so the __packed makes no difference.

Using __packed requires the compiler generate byte loads/store
with shifts (etc) on many architectures and should really be avoided
unless it is absolutely needed for binary compatibility.

Even then if the problem is a 64bit field that only needs to be
32bit aligned (as is common for some compat32 code) then the 64bit
fields should be marked as being 32bit aligned.

	David

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