On Thu, Aug 13, 2009 at 03:38:43PM +0200, Arnd Bergmann wrote: > On Thursday 13 August 2009, Michael S. Tsirkin wrote: > > On Wed, Aug 12, 2009 at 07:59:47PM +0200, Arnd Bergmann wrote: > > > The trick is to swap the virtqueues instead. virtio-net is actually > > > mostly symmetric in just the same way that the physical wires on a > > > twisted pair ethernet are symmetric (I like how that analogy fits). > > > > You need to really squint hard for it to look symmetric. > > > > For example, for RX, virtio allocates an skb, puts a descriptor on a > > ring and waits for host to fill it in. Host system can not do the same: > > guest does not have access to host memory. > > > > You can do a copy in transport to hide this fact, but it will kill > > performance. > > Yes, that is what I was suggesting all along. The actual copy operation > has to be done by the host transport, which is obviously different from > the guest transport that just calls the host using vring_kick(). > > Right now, the number of copy operations in your code is the same. > You are doing the copy a little bit later in skb_copy_datagram_iovec(), > which is indeed a very nice hack. Changing to a virtqueue based method > would imply that the host needs to add each skb_frag_t to its outbound > virtqueue, which then gets copied into the guests inbound virtqueue. Which is a lot more code than just calling skb_copy_datagram_iovec. > Unfortunately, this also implies that you could no longer simply use the > packet socket interface as you do currently, as I realized only now. > This obviously has a significant impact on your user space interface. > > Arnd <>< And, it will remove our ability to implement zero copy down the road (when raw sockets support it). -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
