On Wed, May 18, 2011 at 01:47:33PM +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. > > One more question: is userspace (or whatever is sending those packets) > denied from modifying passed pages? I assume it is, but just want to > be sure. > > Best Regards, > MichaÅ MirosÅaw Good point. It's not denied in the sense that it still can modify them if it's buggy (the pages might not be read-only). But well-behaved userspace won't modify them until the callback is invoked. That would be a problem if the underlying device is a bridge where we might try to e.g. filter these packets - data can get modified after the filter. We'd have to copy whatever the filter accesses and use the copy - it's rarely the data itself. That's not normally a problem for macvtap connected to a physical NIC, as that already bypasses any and all filtering. But that's another limitation we should note in the comment, and another reason to limit to specific devices. -- 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
