Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Sep 30, 2016 at 02:05:10PM +0200, Maxime Coquelin wrote:
> 
> 
> On 09/29/2016 11:23 PM, Maxime Coquelin wrote:
> > 
> > 
> > On 09/29/2016 10:21 PM, Michael S. Tsirkin wrote:
> > > On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote:
> > > > 
> > > > 
> > > > On 09/29/2016 07:57 PM, Michael S. Tsirkin wrote:
> > > > > On Thu, Sep 29, 2016 at 05:30:53PM +0200, Maxime Coquelin wrote:
> > > > ...
> > > > > > 
> > > > > > Before enabling anything by default, we should first optimize the 1
> > > > > > slot
> > > > > > case. Indeed, micro-benchmark using testpmd in txonly[0] shows ~17%
> > > > > > perf regression for 64 bytes case:
> > > > > >  - 2 descs per packet: 11.6Mpps
> > > > > >  - 1 desc per packet: 9.6Mpps
> > > > > > 
> > > > > > This is due to the virtio header clearing in virtqueue_enqueue_xmit().
> > > > > > Removing it, we get better results than with 2 descs (1.20Mpps).
> > > > > > Since the Virtio PMD doesn't support offloads, I wonder whether we can
> > > > > > just drop the memset?
> > > > > 
> > > > > What will happen? Will the header be uninitialized?
> > > > Yes..
> > > > I didn't look closely at the spec, but just looked at DPDK's and Linux
> > > > vhost implementations. IIUC, the header is just skipped in the two
> > > > implementations.
> > > 
> > > In linux guest skbs are initialized AFAIK. See virtio_net_hdr_from_skb
> > > first thing it does is
> > >         memset(hdr, 0, sizeof(*hdr));
> > 
> > I meant in vhost-net linux implementation, the header is just skipped:
> > 
> > static void handle_tx(struct vhost_net *net)
> > {
> > ...
> >         /* Skip header. TODO: support TSO. */
> >         len = iov_length(vq->iov, out);
> >         iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
> >         iov_iter_advance(&msg.msg_iter, hdr_size);
> > 
> > And the same is done is done in DPDK:
> > 
> > static inline int __attribute__((always_inline))
> > copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
> >           uint16_t max_desc, struct rte_mbuf *m, uint16_t desc_idx,
> >           struct rte_mempool *mbuf_pool)
> > {
> > ...
> >     /*
> >      * A virtio driver normally uses at least 2 desc buffers
> >      * for Tx: the first for storing the header, and others
> >      * for storing the data.
> >      */
> >     if (likely((desc->len == dev->vhost_hlen) &&
> >            (desc->flags & VRING_DESC_F_NEXT) != 0)) {
> >         desc = &descs[desc->next];
> >         if (unlikely(desc->flags & VRING_DESC_F_INDIRECT))
> >             return -1;
> > 
> >         desc_addr = gpa_to_vva(dev, desc->addr);
> >         if (unlikely(!desc_addr))
> >             return -1;
> > 
> >         rte_prefetch0((void *)(uintptr_t)desc_addr);
> > 
> >         desc_offset = 0;
> >         desc_avail  = desc->len;
> >         nr_desc    += 1;
> > 
> >         PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
> >     } else {
> >         desc_avail  = desc->len - dev->vhost_hlen;
> >         desc_offset = dev->vhost_hlen;
> >     }
> 
> Actually, the header is parsed in DPDK vhost implementation.
> But as Virtio PMD provides a zero'ed header, we could just parse
> the header only if VIRTIO_NET_F_NO_TX_HEADER is not negotiated.

host can always skip the header parse if it wants to.
It didn't seem worth it to add branches there but
if I'm wrong, by all means code it up.


> > > 
> > > 
> > > 
> > > > > 
> > > > > The spec says:
> > > > >     The driver can send a completely checksummed packet. In this
> > > > > case, flags
> > > > >     will be zero, and gso_type
> > > > >     will be VIRTIO_NET_HDR_GSO_NONE.
> > > > > 
> > > > > and
> > > > >     The driver MUST set num_buffers to zero.
> > > > >     If VIRTIO_NET_F_CSUM is not negotiated, the driver MUST set
> > > > > flags to
> > > > >     zero and SHOULD supply a fully
> > > > >     checksummed packet to the device.
> > > > > 
> > > > > and
> > > > >     If none of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have
> > > > > been
> > > > >     negotiated, the driver MUST
> > > > >     set gso_type to VIRTIO_NET_HDR_GSO_NONE.
> > > > > 
> > > > > so doing this unconditionally would be a spec violation, but if you see
> > > > > value in this, we can add a feature bit.
> > > > Right it would be a spec violation, so it should be done conditionally.
> > > > If a feature bit is to be added, what about VIRTIO_NET_F_NO_TX_HEADER?
> > > > It would imply VIRTIO_NET_F_CSUM not set, and no GSO features set.
> > > > If negotiated, we wouldn't need to prepend a header.
> > > 
> > > Yes but two points.
> > > 
> > > 1. why is this memset expensive? Is the test completely skipping looking
> > >    at the packet otherwise?
> > Yes.
> > > 
> > > 2. As long as we are doing this, see
> > >     Alignment vs. Networking
> > >     ========================
> > > in Documentation/unaligned-memory-access.txt
> > Thanks, I'll have a look tomorrow.
> 
> I did a rough prototype which removes Tx headers unconditionally, to
> see what gain we could expect. I expect the results to be a little lower
> with no headers in full implementation, as some more checks will have
> to be done.
> 
> For PVP use-case with 0.05% acceptable packets loss:
>  - Original (with headers): 9.43Mpps
>  - Indirect descs: 9.36 Mpps
>  - Prototype (no headers): 10.65Mpps
> 
> For PVP use-case with 0% acceptable packets loss:
>  - Original (with headers): 5.23Mpps
>  - Indirect descs: 7.13 Mpps
>  - Prototype (no headers): 7.92Mpps
> 
> Maxime