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

[Wang, Zhihong]

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu at linux.intel.com]
> Sent: Monday, September 5, 2016 1:25 PM
> To: Wang, Zhihong <zhihong.wang at intel.com>
> Cc: dev at dpdk.org; maxime.coquelin at redhat.com;
> yuanhan.liu at linux.intel.com; thomas.monjalon at 6wind.com;
> stable at dpdk.org
> Subject: Re: [dpdk-stable] [PATCH v4 1/6] vhost: fix windows vm hang
> 
> 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.

Okay, I'll add it in v5.

> 
> > 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 at dpdk.org>
> > Signed-off-by: Zhihong Wang <zhihong.wang at 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.

No it's not, desc_offset is the offset of the current desc only.
That's where the old code goes wrong.

If you take a look at the virtio spec:

/* le32 is used here for ids for padding reasons. */
struct vring_used_elem {
/* Index of start of used descriptor chain. */
le32 id;
/* Total length of the descriptor chain which was written to. */
le32 len;
};

> 
> 	--yliu