Re: [PATCH v2] vhost: Add indirect descriptors support to the TX path

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

On Fri, Sep 23, 2016 at 01:28:05PM -0700, Stephen Hemminger wrote:
> On Fri, 23 Sep 2016 21:31:27 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Fri, Sep 23, 2016 at 11:24:16AM -0700, Stephen Hemminger wrote:
> > > On Fri, 23 Sep 2016 09:16:49 +0200
> > > Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
> > >   
> > > > Indirect descriptors are usually supported by virtio-net devices,
> > > > allowing to dispatch a larger number of requests.
> > > > 
> > > > When the virtio device sends a packet using indirect descriptors,
> > > > only one slot is used in the ring, even for large packets.
> > > > 
> > > > The main effect is to improve the 0% packet loss benchmark.
> > > > A PVP benchmark using Moongen (64 bytes) on the TE, and testpmd
> > > > (fwd io for host, macswap for VM) on DUT shows a +50% gain for
> > > > zero loss.
> > > > 
> > > > On the downside, micro-benchmark using testpmd txonly in VM and
> > > > rxonly on host shows a loss between 1 and 4%.i But depending on
> > > > the needs, feature can be disabled at VM boot time by passing
> > > > indirect_desc=off argument to vhost-user device in Qemu.
> > > > 
> > > > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>  
> > > 
> > > What about supporting VIRTIO_F_ANY_LAYOUT?  
> > 
> > I thought it's already supported.
> > That's required by virtio 1 and dpdk claims support for that.
> > 
> 
> I don't see the flag set in the DPDK vhost driver feature bits
> (at least in the source). 
> 
> /* Features supported by this lib. */
> #define VHOST_SUPPORTED_FEATURES ((1ULL << VIRTIO_NET_F_MRG_RXBUF) | \
> 				(1ULL << VIRTIO_NET_F_CTRL_VQ) | \
> 				(1ULL << VIRTIO_NET_F_CTRL_RX) | \
> 				(1ULL << VIRTIO_NET_F_GUEST_ANNOUNCE) | \
> 				(VHOST_SUPPORTS_MQ)            | \
> 				(1ULL << VIRTIO_F_VERSION_1)   | \
> 				(1ULL << VHOST_F_LOG_ALL)      | \
> 				(1ULL << VHOST_USER_F_PROTOCOL_FEATURES) | \
> 				(1ULL << VIRTIO_NET_F_HOST_TSO4) | \
> 				(1ULL << VIRTIO_NET_F_HOST_TSO6) | \
> 				(1ULL << VIRTIO_NET_F_CSUM)    | \
> 				(1ULL << VIRTIO_NET_F_GUEST_CSUM) | \
> 				(1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
> 				(1ULL << VIRTIO_NET_F_GUEST_TSO6))


I see. It's implied by VERSION_1 in fact.
In other words if VERSION_1 is negotiated then bit 27
isn't ANY_LAYOUT, it's in fact reserved.

-- 
MST