On Wed, Feb 20, 2019 at 10:34:32AM +0800, Jason Wang wrote: > > On 2019/2/20 上午9:54, Wei Xu wrote: > >On Tue, Feb 19, 2019 at 09:09:33PM +0800, Jason Wang wrote: > >>On 2019/2/19 下午6:51, Wei Xu wrote: > >>>On Tue, Feb 19, 2019 at 03:23:01PM +0800, Jason Wang wrote: > >>>>On 2019/2/14 下午12:26, w...@redhat.com wrote: > >>>>>From: Wei Xu <w...@redhat.com> > >>>>> > >>>>>This is a helper for packed ring. > >>>>> > >>>>>To support packed ring, the head descriptor in a chain should be updated > >>>>>lastly since no 'avail_idx' like in packed ring to explicitly tell the > >>>>>driver side that all payload is ready after having done the chain, so > >>>>>the head is always visible immediately. > >>>>> > >>>>>This patch fills the header after done all the other ones. > >>>>> > >>>>>Signed-off-by: Wei Xu <w...@redhat.com> > >>>>It's really odd to workaround API issue in the implementation of device. > >>>>Please introduce batched used updating helpers instead. > >>>Can you elaborate a bit more? I don't get it as well. > >>> > >>>The exact batch as vhost-net or dpdk pmd is not supported by userspace > >>>backend. The change here is to keep the header descriptor updated at > >>>last in case of a chaining descriptors and the helper might not help > >>>too much. > >>> > >>>Wei > >> > >>Of course we can add batching support why not? > >It is always good to improve performance with anything, while probably > >this could be done in another separate batch, also we need to bear > >in mind that usually qemu userspace backend is not the first option for > >performance oriented user. > > > The point is to hide layout specific things from device emulation. If it > helps for performance, it could be treated as a good byproduct. > > > > > >AFAICT, virtqueue_fill() is a generic API for all relevant userspace virtio > >devices that do not support batching , without touching virtqueue_fill(), > >supporting batching changes the meaning of the parameter 'idx' which should > >be kept overall. > > > >To fix it, I got two proposals so far: > >1). batching support(two APIs needed to keep compatibility) > >2). save a head elem for a vq instead of caching an array of elems like > >vhost, > > and introduce a new API(virtqueue_chain_fill()) functioning with an > > additional parameter 'more' to the current virtqueue_fill() to indicate > > if > > there are more descriptor(s) coming in a chain. > > > >Either way it changes the API somehow and it does not seem to be clean and > >clear > >as wanted. > > > It's as simple as accepting an array of elems in e.g > virtqueue_fill_batched()?
It is trivial for both, my concern is an array elements need to be allocated dynamically due to vq size which no any other devices are using, a head would be enough for 2. > > > > > >Any better idea? > > > >>Your code assumes the device know the virtio layout specific assumption > >>whih breaks the layer. Device should not care about the actual layout. > >> > >Good point, but anyway, change to virtio-net receiving code path is > >unavoidable to support split and packed rings, and batching is like a new > >feature somehow. > > > It's ok to change the code as a result of introducing of generic helper but > it's bad to change to code for working around a bad API. Agree. Wei > > Thanks > > > > > >Wei > >>Thanks > >> > >> > >>>>Thanks > >>>> > >>>> > >>>>>--- > >>>>> hw/net/virtio-net.c | 11 ++++++++++- > >>>>> 1 file changed, 10 insertions(+), 1 deletion(-) > >>>>> > >>>>>diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > >>>>>index 3f319ef..330abea 100644 > >>>>>--- a/hw/net/virtio-net.c > >>>>>+++ b/hw/net/virtio-net.c > >>>>>@@ -1251,6 +1251,8 @@ static ssize_t > >>>>>virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf, > >>>>> struct virtio_net_hdr_mrg_rxbuf mhdr; > >>>>> unsigned mhdr_cnt = 0; > >>>>> size_t offset, i, guest_offset; > >>>>>+ VirtQueueElement head; > >>>>>+ int head_len = 0; > >>>>> if (!virtio_net_can_receive(nc)) { > >>>>> return -1; > >>>>>@@ -1328,7 +1330,13 @@ static ssize_t > >>>>>virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf, > >>>>> } > >>>>> /* signal other side */ > >>>>>- virtqueue_fill(q->rx_vq, elem, total, i++); > >>>>>+ if (i == 0) { > >>>>>+ head_len = total; > >>>>>+ head = *elem; > >>>>>+ } else { > >>>>>+ virtqueue_fill(q->rx_vq, elem, len, i); > >>>>>+ } > >>>>>+ i++; > >>>>> g_free(elem); > >>>>> } > >>>>>@@ -1339,6 +1347,7 @@ static ssize_t > >>>>>virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf, > >>>>> &mhdr.num_buffers, sizeof mhdr.num_buffers); > >>>>> } > >>>>>+ virtqueue_fill(q->rx_vq, &head, head_len, 0); > >>>>> virtqueue_flush(q->rx_vq, i); > >>>>> virtio_notify(vdev, q->rx_vq); >