netdev - Re: [PATCH 2/2] net: add driver for Netronome NFP4000/NFP6000 NIC VFs

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Date:	Tue, 27 Oct 2015 11:16:03 +0000
From:	Jakub Kicinski <jakub.kicinski@...ronome.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, rolf.neugebauer@...ronome.com,
	jason.mcmullan@...ronome.com, simon.horman@...ronome.com
Subject: Re: [PATCH 2/2] net: add driver for Netronome NFP4000/NFP6000 NIC
 VFs

On Mon, 26 Oct 2015 18:23:07 -0700 (PDT), David Miller wrote:
> From: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Date: Fri, 23 Oct 2015 19:58:11 +0100
> 
> > +struct nfp_net_tx_buf {
> > +	struct sk_buff *skb;
> > +	dma_addr_t dma_addr;
> > +	short int fidx;
> > +	u16 pkt_cnt;
> > +	u32 real_len;
> > +};
> 
> This packs very poorly, and has a lot of padding holes.  Better ordering
> would be:
> 
> struct nfp_net_tx_buf {
> 	struct sk_buff *skb;
> 	dma_addr_t dma_addr;
> 	u32 real_len;
> 	short int fidx;
> 	u16 pkt_cnt;
> };

Seems to pack fine on x86:

struct nfp_net_tx_buf {
        struct sk_buff *           skb;                  /*     0     8 */
        dma_addr_t                 dma_addr;             /*     8     8 */
        short int                  fidx;                 /*    16     2 */
        u16                        pkt_cnt;              /*    18     2 */
        u32                        real_len;             /*    20     4 */

        /* size: 24, cachelines: 1, members: 5 */
        /* last cacheline: 24 bytes */
};

Are my packing skills deceiving me?  Maybe I'll try to build the driver
for more esoteric architectures.
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