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 11:06:00 +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> <1305675865.10756.63.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Michael S. Tsirkin" , Ben Hutchings , David Miller , Eric Dumazet , Avi Kivity , Arnd Bergmann , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Shirley Ma Return-path: In-Reply-To: <1305675865.10756.63.camel@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org W dniu 18 maja 2011 01:44 u=BFytkownik Shirley Ma = napisa=B3: > On Wed, 2011-05-18 at 00:58 +0200, Micha=B3 Miros=B3aw wrote: >> W dniu 18 maja 2011 00:28 u=BFytkownik Shirley Ma >> napisa=B3: >> > On Tue, 2011-05-17 at 23:48 +0200, Micha=B3 Miros=B3aw wrote: >> >> 2011/5/17 Shirley Ma : >> >> > 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 lowe= r >> >> device? >> >> >> >> Is there any other restriction besides requiring driver to not >> recycle >> >> the skb? Are there any drivers that recycle TX skbs? >> > Not more other restrictions, skb clone is OK. pskb_expand_head() >> looks >> > OK to me from code review. >> >> > Currently there is no drivers recycle TX skbs. >> >> So why do you require the target device to have some flags at all? > We could use macvtap to check lower device HIGHDMA to enable zero-cop= y, > but I am not sure whether it is sufficient. If it's sufficient then w= e > don't need to use a new flag here. To be safe, it's better to use a n= ew > flag to enable each device who can pass zero-copy test. >> Do I understand correctly, that this zero-copy feature is about >> packets received from VMs? > Yes, packets sent from VMs, and received in local host for TX zero-co= py > here. What is the zero-copy test? On some arches the HIGHDMA is not needed at all so might be not enabled on anything. It looks like the correct test would be per-packet check of !illegal_highdma() or maybe NETIF_F_SG as returned from harmonize_features(). For virtual devices or other software forwarding this might lead to skb_linearize() in some cases, but is it that bad? Best Regards, Micha=B3 Miros=B3aw