[dpdk-dev] [PATCH] virtio: use zeroed memory for simple TX header

[Rich Lane]

On Mon, Apr 4, 2016 at 1:05 PM, Yuanhan Liu <yuanhan.liu at linux.intel.com>
wrote:

> On Mon, Apr 04, 2016 at 03:13:37PM +0200, Thomas Monjalon wrote:
> > Huawei, Yuanhan, any comment?
> >
> > 2016-03-31 13:01, Rich Lane:
> > >                             vq->vq_ring.desc[i + mid_idx].next = i;
> > >                             vq->vq_ring.desc[i + mid_idx].addr =
> > > -                                   vq->virtio_net_hdr_mem +
> > > -                                           i * vq->hw->vtnet_hdr_size;
> > > +                                   vq->virtio_net_hdr_mem;
>
> I could be wrong, but this looks like a special case when i == 0,
> which is by no way that zeroed memory is guaranteed? Huawei, do
> you have time to check this patch?


This bug exists because the type of the objects pointed to by
virtio_net_hdr_mem changed in 6dc5de3a (virtio: use indirect ring
elements), but because it isn't a C pointer the compiler didn't catch the
type mismatch. We could also fix it with:

    vq->virtio_net_hdr_mem + i * sizeof(struct virtio_tx_region) +
offsetof(struct virtio_tx_region, tx_hdr)

Given that tx_hdr is the first member in struct virtio_tx_region, and using
a single header optimizes cache use, that simplifies to the code in my
patch. The virtio-net header is never written to by simple TX so it remains
zeroed.

I can respin the patch using offsetof if that's preferred.

Note that right now virtio simple TX is broken with DPDK vhost due to the
flood of error messages.