From mboxrd@z Thu Jan  1 00:00:00 1970
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Subject: Re: [PATCH 0/6] vhost: add Tx zero copy support
Date: Mon, 10 Oct 2016 16:03:39 +0800
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Cc: dev@dpdk.org, Maxime Coquelin <maxime.coquelin@redhat.com>
To: linhaifeng <haifeng.lin@huawei.com>
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On Sun, Oct 09, 2016 at 06:46:44PM +0800, linhaifeng wrote:
> 在 2016/8/23 16:10, Yuanhan Liu 写道:
> > The basic idea of Tx zero copy is, instead of copying data from the
> > desc buf, here we let the mbuf reference the desc buf addr directly.
> 
> Is there problem when push vlan to the mbuf which reference the desc buf addr directly?

Yes, you can't do that when zero copy is enabled, due to following code
piece:

    +               if (unlikely(dev->dequeue_zero_copy && (hpa = gpa_to_hpa(dev,
    +                                       desc->addr + desc_offset, cpy_len)))) {
    +                       cur->data_len = cpy_len;
==> +                       cur->data_off = 0;
    +                       cur->buf_addr = (void *)(uintptr_t)desc_addr;
    +                       cur->buf_physaddr = hpa;

The marked line basically makes the mbuf has no headroom to use.

	--yliu

> We know if guest use virtio_net(kernel) maybe skb has no headroom.