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Date: Mon, 10 Oct 2016 16:03:39 +0800
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: linhaifeng <haifeng.lin@huawei.com>
Cc: dev@dpdk.org, Maxime Coquelin <maxime.coquelin@redhat.com>
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Subject: Re: [dpdk-dev] [PATCH 0/6] vhost: add Tx zero copy support
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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.