On Wed, Aug 23, 2017 at 11:20:45AM -0400, Willem de Bruijn wrote: > > Please let me make sure if I understand it correctly: > > * always do copy with skb_orphan_frags_rx as Willem mentioned in the earlier > > post, before the xmit_skb as opposed to my original patch, is safe but too > > costly so cannot be adopted. > > One more point about msg_zerocopy in the guest. This does add new allocation > limits on optmem and locked pages rlimit. > > Hitting these should be extremely rare. The tcp small queues limit normally > throttles well before this. > > Virtio-net is an exception because it breaks the tsq signal by calling > skb_orphan before transmission. > > As a result hitting these limits is more likely here. But, in this edge case > the > sendmsg call will not block, either, but fail with -ENOBUFS. The caller can > send without zerocopy to make forward progress and > trigger free_old_xmit_skbs from start_xmit. > > > * as a generic solution, if we were to somehow overcome the safety issue, > > track > > the delay and do copy if some threshold is reached could be an answer, but > > it's > > hard for now.> * so things like the current vhost-net implementation of > > deciding whether or not > > to do zerocopy beforehand referring the zerocopy tx error ratio is a point > > of > > practical compromise. > > The fragility of this mechanism is another argument for switching to tx napi > as default. > > Is there any more data about the windows guest issues when completions > are not queued within a reasonable timeframe? What is this timescale and > do we really need to work around this.
I think it's pretty large, many milliseconds. But I wonder what do you mean by "work around". Using buffers within limited time frame sounds like a reasonable requirement to me. Neither do I see why would using tx interrupts within guest be a work around - AFAIK windows driver uses tx interrupts. > That is the only thing keeping us from removing the HoL blocking in vhost-net > zerocopy. We don't enable network watchdog on virtio but we could and maybe should. -- MST
