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, Michał Mirosław 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. For 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. Is it that you mean keeping a reference after all skbs pointing to the pages are released? >> 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. Best Regards, Michał Mirosław -- 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
