linux-kernel - Re: [PATCH v2 16/16] net: Add support for networking over Thunderbolt cable

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Date:   Wed, 27 Sep 2017 16:42:38 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     David Miller <davem@...emloft.net>
Cc:     gregkh@...uxfoundation.org, andreas.noever@...il.com,
        michael.jamet@...el.com, yehezkel.bernat@...el.com,
        amir.jer.levy@...el.com, Mario.Limonciello@...l.com,
        lukas@...ner.de, andriy.shevchenko@...ux.intel.com, andrew@...n.ch,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 16/16] net: Add support for networking over
 Thunderbolt cable

On Tue, Sep 26, 2017 at 09:47:21PM -0700, David Miller wrote:
> From: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Date: Mon, 25 Sep 2017 14:07:38 +0300
> 
> > +struct thunderbolt_ip_header {
> > +	u32 route_hi;
> > +	u32 route_lo;
> > +	u32 length_sn;
> > +	uuid_t uuid;
> > +	uuid_t initiator_uuid;
> > +	uuid_t target_uuid;
> > +	u32 type;
> > +	u32 command_id;
> > +} __packed;
> 
> Again, the __packed attribute should not be necessary and needs to be
> removed.

OK, will do.

> > +static void tbnet_pull_tail(struct sk_buff *skb)
> > +{
> > +	skb_frag_t *frag = &skb_shinfo(skb)->frags[0];
> > +	unsigned int pull_len;
> > +	void *hdr;
> > +
> > +	hdr = skb_frag_address(frag);
> > +	pull_len = eth_get_headlen(hdr, TBNET_RX_HDR_SIZE);
> > +
> > +	/* Align pull length to size of long to optimize memcpy performance */
> > +	skb_copy_to_linear_data(skb, hdr, ALIGN(pull_len, sizeof(long)));
> 
> You do not need to copy here, instead you can build SKB's where the
> skb->data points directly at the head of your first frag page memory.
> 
> See build_skb().
> 
> > +		skb = net->skb;
> > +		if (!skb) {
> > +			skb = netdev_alloc_skb_ip_align(net->dev,
> > +							TBNET_RX_HDR_SIZE);
> > +			net->skb = skb;
> > +		}
> > +		if (!skb)
> > +			break;
> > +
> > +		/* Single small buffer we can copy directly to the
> > +		 * header part of the skb.
> > +		 */
> > +		if (hdr->frame_count == 1 && frame_size <= TBNET_RX_HDR_SIZE) {
> 
> Here you would use build_skb() instead of netdev_alloc_skb*() for the first
> frag, and keep the existing code tacking on subsequent frags using
> skb_add_Rx_frag().

I'm reading kernel-doc of build_skb() (or rather __build_skb()) and it
says caller needs to reserve head room of NET_SKB_PAD and then make sure
there is space for SKB_DATA_ALIGN(skb_shared_info).

Now, in case of ThunderboltIP frames, they look like this:

  +---------+
  | hdr     | 12 bytes
  +---------+
  | data    | 4096 - 12 = 4084 bytes
  |         |
  |         |
  +---------+

A packet can consist of multiple frames where each have the 12-byte
header and 4084 bytes of TSO/LRO payload except the last one which can
be smaller than 4084.

Using build_skb() then would require to allocate larger buffer, that
includes NET_SKB_PAD + SKB_DATA_ALIGN(skb_shared_info) and that exceeds
page size. Is this something supported by build_skb()? It was not clear
to me based on the code and other users of build_skb() but I may be
missing something.

> > +	ret = register_netdev(dev);
> > +	if (ret) {
> > +		free_netdev(dev);
> > +		return ret;
> > +	}
> > +
> > +	net->handler.uuid = &tbnet_svc_uuid;
> > +	net->handler.callback = tbnet_handle_packet,
> > +	net->handler.data = net;
> > +	tb_register_protocol_handler(&net->handler);
> > +
> > +	tb_service_set_drvdata(svc, net);
> 
> There could be races here.
> 
> At the exact moment you call register_netdev(), your device can be
> brought UP, packets transmitted, etc.  You entire set of driver code
> paths can be executed.
> 
> The rest of those initializations after register_netdev() probably
> are needed by the rest of the driver to function properly, so may
> need to happen before register_netdev() publishes the device to the
> entire world.

You're right. I'll change the ordering so that register_netdev() happens
last.

Thanks!