From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?ISO-8859-2?Q?Micha=B3_Miros=B3aw?= Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice Date: Wed, 18 May 2011 13:47:33 +0200 Message-ID: References: <1305574128.3456.23.camel@localhost.localdomain> <1305574518.2885.25.camel@bwh-desktop> <1305574680.3456.33.camel@localhost.localdomain> <1305575253.2885.28.camel@bwh-desktop> <20110516211459.GE18148@redhat.com> <1305588738.3456.65.camel@localhost.localdomain> <1305671318.10756.49.camel@localhost.localdomain> <20110518103819.GL7589@redhat.com> <20110518111734.GO7589@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Shirley Ma , Ben Hutchings , David Miller , Eric Dumazet , Avi Kivity , Arnd Bergmann , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20110518111734.GO7589@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org W dniu 18 maja 2011 13:17 u=BFytkownik Michael S. Tsirkin napisa=B3: > On Wed, May 18, 2011 at 01:10:50PM +0200, Micha=B3 Miros=B3aw wrote: >> 2011/5/18 Michael S. Tsirkin : >> > On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote: >> >> On Tue, 2011-05-17 at 23:48 +0200, Micha=B3 Miros=B3aw wrote: >> >> > 2011/5/17 Shirley Ma : >> >> > > Hello Michael, >> >> > > >> >> > > Looks like to use a new flag requires more time/work. I am th= inking >> >> > > whether we can just use HIGHDMA flag to enable zero-copy in m= acvtap >> >> > to >> >> > > avoid the new flag for now since mavctap uses real NICs as lo= wer >> >> > 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. =A0For example virtio-net doesn't limit the t= ime 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 ca= n >> 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, Micha=B3 Miros=B3aw