On Thursday 09 October 2008 06:34:59 Mark McLoughlin wrote: > From: Herbert Xu <herbert.xu@xxxxxxxxxx> > > If segmentation offload is enabled by the host, we currently allocate > maximum sized packet buffers and pass them to the host. This uses up > 20 ring entries, allowing us to supply only 20 packet buffers to the > host with a 256 entry ring. This is a huge overhead when receiving > small packets, and is most keenly felt when receiving MTU sized > packets from off-host. Hi Mark! There are three approaches we should investigate before adding YA feature. Obviously, we can simply increase the number of ring entries. Secondly, we can put the virtio_net_hdr at the head of the skb data (this is also worth considering for xmit I think if we have headroom) and drop MAX_SKB_FRAGS which contains a gratuitous +2. Thirdly, we can try to coalesce contiguous buffers. The page caching scheme we have might help here, I don't know. Maybe we should be explicitly trying to allocate higher orders. Now, that said, we might need this anyway. But let's try the easy things first? (Or as well...) > The size of the logical buffer is > returned to the guest rather than the size of the individual smaller > buffers. That's a virtio transport breakage: can you use the standard virtio mechanism, just put the extended length or number of extra buffers inside the virtio_net_hdr? That makes more sense to me. > Make use of this support by supplying single page receive buffers to > the host. On receive, we extract the virtio_net_hdr, copy 128 bytes of > the payload to the skb's linear data buffer and adjust the fragment > offset to point to the remaining data. This ensures proper alignment > and allows us to not use any paged data for small packets. If the > payload occupies multiple pages, we simply append those pages as > fragments and free the associated skbs. > + char *p = page_address(skb_shinfo(skb)->frags[0].page); ... > + memcpy(hdr, p, sizeof(*hdr)); > + p += sizeof(*hdr); I think you need kmap_atomic() here to access the page. And yes, that will effect performance :( A few more comments moved from the patch header into the source wouldn't go astray, but I'm happy to do that myself (it's been on my TODO for a while). Thanks! Rusty. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
