Re: [PATCH] virtio: use zeroed memory for simple TX header

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

On Mon, Apr 04, 2016 at 03:57:11PM -0700, Rich Lane wrote:
> On Mon, Apr 4, 2016 at 1:05 PM, Yuanhan Liu <yuanhan.liu@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.

It does. However, it hurts readability.

> 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.

Yes, please. In such way, we could also align with the setting up code
at virtio_dev_queue_setup().

BTW, I have one question: will simple Tx work with indirect buf
enabled?

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

Yes, we need the fix, and thanks for the catching. BTW, it's a
regression fix, you'd better add a Fixline into your commit log.

	--yliu