Re: [PATCH 0/6] vhost: add Tx zero copy support

On Sun, Oct 09, 2016 at 06:46:44PM +0800, linhaifeng wrote:

在 2016/8/23 16:10, Yuanhan Liu 写道:

quoted

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.