On Thu, 2011-05-26 at 11:49 +0300, Michael S. Tsirkin wrote: > 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? That's I have done already. Patch is going out for review. > > 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? There is no problem to use it, Mahesh is working on this patch. I just want to remove macvtap/vhost zero-copy patch dependency. Thanks Shirley -- 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
