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. > > Best Regards, > MichaÅ MirosÅaw 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. -- 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
