W dniu 18 maja 2011 13:17 użytkownik Michael S. Tsirkin <mst@xxxxxxxxxx> napisał: > On Wed, May 18, 2011 at 01:10:50PM +0200, Michał Mirosław 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, 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. > 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, 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
