Re: [dpdk-stable] [PATCH v4 1/6] vhost: fix windows vm hang

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Mon, Aug 29, 2016 at 11:35:59PM -0400, Zhihong Wang wrote:
> This patch fixes a Windows VM compatibility issue in DPDK 16.07 vhost code,
> which causes the guest to hang once any packets are enqueued when mrg_rxbuf
> is turned on.

This commit log lacks two important pieces: why does the hang happen and
how does your patch fix it.

> How to test?
> 
>  1. Start testpmd in the host with a vhost port.
> 
>  2. Start a Windows VM image with qemu and connect to the vhost port.
> 
>  3. Start io forwarding with tx_first in host testpmd.
> 
> For 16.07 code, the Windows VM will hang once any packets are enqueued.
> 
> Cc: <stable@dpdk.org>
> Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
> ---
>  lib/librte_vhost/vhost_rxtx.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
> index 08a73fd..5806f99 100644
> --- a/lib/librte_vhost/vhost_rxtx.c
> +++ b/lib/librte_vhost/vhost_rxtx.c
> @@ -384,6 +384,8 @@ copy_mbuf_to_desc_mergeable(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  	uint16_t start_idx = vq->last_used_idx;
>  	uint16_t cur_idx = start_idx;
>  	uint64_t desc_addr;
> +	uint32_t desc_chain_head;
> +	uint32_t desc_chain_len;

What's the point of introducing "desc_chain_len"? It has the same value
of desc_offset.

	--yliu