Re: [PATCH net-next v3 00/13] virtio: support packed ring

[Jason Wang]

On 2018/11/21 äå8:42, Tiwei Bie wrote:
> On Wed, Nov 21, 2018 at 07:20:27AM -0500, Michael S. Tsirkin wrote:
> > On Wed, Nov 21, 2018 at 06:03:17PM +0800, Tiwei Bie wrote:
> > > Hi,
> > >
> > > This patch set implements packed ring support in virtio driver.
> > >
> > > A performance test between pktgen (pktgen_sample03_burst_single_flow.sh)
> > > and DPDK vhost (testpmd/rxonly/vhost-PMD) has been done, I saw
> > > ~30% performance gain in packed ring in this case.
> > Thanks a lot, this is very exciting!
> > Dave, given the holiday, attempts to wrap up the 1.1 spec and the
> > patchset size I would very much appreciate a bit more time for
> > review. Say until Nov 28?
> >
> > > To make this patch set work with below patch set for vhost,
> > > some hacks are needed to set the _F_NEXT flag in indirect
> > > descriptors (this should be fixed in vhost):
> > >
> > > https://lkml.org/lkml/2018/7/3/33
> > Could you pls clarify - do you mean it doesn't yet work with vhost
> > because of a vhost bug, and to test it with the linked patches
> > you had to hack in _F_NEXT? Because I do not see _F_NEXT
> > in indirect descriptors in this patch (which is fine).
> > Or did I miss it?
> You didn't miss anything. :)
>
> I think it's a small bug in vhost, which Jason may fix very
> quickly, so I didn't post it. Below is the hack I used:


Good catch. I didn't notice the subtle difference since split ring 
requires for it.

Let me fix it in next version.

Thanks.


> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cd7e755484e3..42faea7d8cf8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -980,6 +980,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>   	unsigned int i, n, err_idx;
>   	u16 head, id;
>   	dma_addr_t addr;
> +	int c = 0;
>   
>   	head = vq->packed.next_avail_idx;
>   	desc = alloc_indirect_packed(total_sg, gfp);
> @@ -1001,8 +1002,9 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>   			if (vring_mapping_error(vq, addr))
>   				goto unmap_release;
>   
> -			desc[i].flags = cpu_to_le16(n < out_sgs ?
> -						0 : VRING_DESC_F_WRITE);
> +			desc[i].flags = cpu_to_le16((n < out_sgs ?
> +						0 : VRING_DESC_F_WRITE) |
> +				    (++c == total_sg ? 0 : VRING_DESC_F_NEXT));
>   			desc[i].addr = cpu_to_le64(addr);
>   			desc[i].len = cpu_to_le32(sg->length);
>   			i++;