On Wed, May 25, 2011 at 03:49:40PM -0700, Shirley Ma wrote: > On Fri, 2011-05-20 at 02:41 +0300, Michael S. Tsirkin wrote: > > So the requirements are > > - data must be released in a timely fashion (e.g. unlike virtio-net > > tun or bridge) > The current patch doesn't enable tun zero-copy. tun will copy data It's > not an issue now. > We can disallow macvtap attach to bridge when > zero-copy is enabled. Attach macvtap to a tun device though. Or e.g. veth device ... So there should be so generic way to disable zerocopy. It can either be a whitelist or a blacklist. > > > - SG support > > - HIGHDMA support (on arches where this makes sense) > > This can be checked by device flags. OK, but pls note that SG can get turned off dynamically. > > - no filtering based on data (data is mapped in guest) > > > - on fast path no calls to skb_copy, skb_clone, pskb_copy, > > pskb_expand_head as these are slow > > Any calls to skb_copy, skb_clone, pskb_copy, pskb_expand_head will do a > copy. The performance should be the same as none zero-copy case before. I'm guessing a copy is cheaper than get_user_pages+copy+put_page. But maybe not by much. Care checking that? > I have done/tested the patch V6, will send it out for review tomorrow. > > I am looking at where there are some cases, skb remains the same for > filtering. To reliably filter on data I think we'll need to copy it first, otherwise guest can change it. Most filters only look at the header though. > > First 2 requirements are a must, all other requirements > > are just dependencies to make sure zero copy will be faster > > than non zero copy. > > Using a new feature bit is probably the simplest approach to > > this. macvtap on top of most physical NICs most likely works > > correctly so it seems a bit more work than it needs to be, > > but it's also the safest one I think ... > > For "macvtap/vhost zero-copy" we can use SG & HIGHDMA to enable it, it > looks safe to me once patching skb_copy, skb_clone, pskb_copy, > pskb_expand_head. > > To extend zero-copy in other usages, we can have a new feature bit > later. > > Is that reasonable? > > Thanks > Shirley Is the problem is extra work needed to extend feature bits? -- 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
