Re: copyless virtio net thoughts?

[Avi Kivity <avi@redhat.com>]

Chris Wright wrote:
> There's been a number of different discussions re: getting copyless virtio
> net (esp. for KVM).  This is just a poke in that general direction to
> stir the discussion.  I'm interested to hear current thoughts

I believe that copyless networking is absolutely essential.

For transmit, copyless is needed to properly support sendfile() type 
workloads - http/ftp/nfs serving.  These are usually high-bandwidth, 
cache-cold workloads where a copy is most expensive.

For receive, the guest will almost always do an additional copy, but it 
will most likely do the copy from another cpu.  Xen netchannel2 
mitigates this somewhat by having the guest request the hypervisor to 
perform the copy when the rx interrupt is processed, but this may still 
be too early (the packet may be destined to a process that is on another 
vcpu), and the extra hypercall is expensive.

In my opinion, it would be ideal to linux-aio enable taps and packet 
sockets.  io_submit() allows submitting multiple buffers in one syscall 
and supports scatter/gather.  io_getevents() supports dequeuing multiple 
packet completions in one syscall.

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