On Wed, May 18, 2011 at 01:40:29PM +0200, MichaÅ MirosÅaw wrote: > W dniu 18 maja 2011 13:17 uÅytkownik Michael S. Tsirkin > <mst@xxxxxxxxxx> napisaÅ: > > On Wed, May 18, 2011 at 01:10:50PM +0200, MichaÅ MirosÅaw wrote: > >> 2011/5/18 Michael S. Tsirkin <mst@xxxxxxxxxx>: > >> > On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote: > >> >> On Tue, 2011-05-17 at 23:48 +0200, MichaÅ MirosÅaw wrote: > >> >> > 2011/5/17 Shirley Ma <mashirle@xxxxxxxxxx>: > >> >> > > Hello Michael, > >> >> > > > >> >> > > Looks like to use a new flag requires more time/work. I am thinking > >> >> > > whether we can just use HIGHDMA flag to enable zero-copy in macvtap > >> >> > to > >> >> > > avoid the new flag for now since mavctap uses real NICs as lower > >> >> > device? > >> >> > > >> >> > Is there any other restriction besides requiring driver to not recycle > >> >> > the skb? Are there any drivers that recycle TX skbs? > >> > > >> > Not just recycling skbs, keeping reference to any of the pages in the > >> > skb. Another requirement is to invoke the callback > >> > in a timely fashion. ÂFor example virtio-net doesn't limit the time until > >> > that happens (skbs are only freed when some other packet is > >> > transmitted), so we need to avoid zcopy for such (nested-virt) > >> > scenarious, right? > >> > >> Hmm. But every hardware driver supporting SG will keep reference to > >> the pages until the packet is sent (or DMA'd to the device). This can > >> take a long time if hardware queue happens to stall for some reason. > > > > That's a fundamental property of zero copy transmit. > > You can't let the application/guest reuse the memory until > > no one looks at it anymore. > > > >> Is it that you mean keeping a reference after all skbs pointing to the > >> pages are released? > > No one should reference the pages after the callback is invoked, yes. > > >> >> Not more other restrictions, skb clone is OK. pskb_expand_head() looks > >> >> OK to me from code review. > >> > Hmm. pskb_expand_head calls skb_release_data while keeping > >> > references to pages. How is that ok? What do I miss? > >> It's making copy of the skb_shinfo earlier, so the pages refcount > >> stays the same. > > Exactly. But the callback is invoked so the guest thinks it's ok to > > change this memory. If it does a corrupted packet will be sent out. > > Hmm. I tool a quick look at skb_clone(), and it looks like this > sequence will break this scheme: > > skb2 = skb_clone(skb...); > kfree_skb(skb) or pskb_expand_head(skb); /* callback called */ > [use skb2, pages still referenced] > kfree_skb(skb); /* callback called again */ > This sequence is common in bridge, might be in other places. > > Maybe this ubuf thing should just track clones? This will make it work > on all devices then. > > Best Regards, > MichaÅ MirosÅaw Well bridge has the problem that packet might get anywhere and it's really hard to track. Same for tun - it can get queued forever. veth, loopback are all a problem I think. IOW we really want to limit this to real physical NICs which mostly all DTRT. Whitelisting them with a new flag is likely the most concervative approach, no? -- 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
