On Thu, May 19, 2011 at 12:42:49PM -0700, Shirley Ma wrote: > On Wed, 2011-05-18 at 10:00 -0700, Shirley Ma wrote: > > On Wed, 2011-05-18 at 19:51 +0300, Michael S. Tsirkin wrote: > > > > > Yes, I agree. I think for tcpdump, we really need to copy the > > > data > > > > > anyway, to avoid guest changing it in between. So we do that > > and > > > then > > > > > use the copy everywhere, release the old one. Hmm? > > > > > > > > Yes. Old one use zerocopy, new one use copy data. > > > > > > > > Thanks > > > > Shirley > > > > > > No, that's wrong, as they might become different with a > > > malicious guest. As long as we copied already, lets realease > > > the data and have everyone use the copy. > > > > Ok, I will patch pskb_expand_head to test it out. > > I am patching skb_copy, skb_clone, pskb_copy, pskb_expand_head to > convert a zero-copy skb to a copy skb to avoid this kind of issue. > > This overhead won't impact macvtap/vhost TX zero-copy normally. > > Shirley OK, that will handle packet socket at least in that it won't crash :) So the requirements are - data must be released in a timely fashion (e.g. unlike virtio-net tun or bridge) - no filtering based on data (data is mapped in guest) - SG support - HIGHDMA support (on arches where this makes sense) - on fast path no calls to skb_copy, skb_clone, pskb_copy, pskb_expand_head as these are slow 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 ... -- 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
