From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice Date: Wed, 18 May 2011 14:17:34 +0300 Message-ID: <20110518111734.GO7589@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 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: =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= Return-path: Content-Disposition: inline In-Reply-To: Sender: kvm-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, May 18, 2011 at 01:10:50PM +0200, Micha=C5=82 Miros=C5=82aw wro= te: > 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=C5=82 Miros=C5=82aw wrote= : > >> > 2011/5/17 Shirley Ma : > >> > > Hello Michael, > >> > > > >> > > Looks like to use a new flag requires more time/work. I am thi= nking > >> > > whether we can just use HIGHDMA flag to enable zero-copy in ma= cvtap > >> > to > >> > > avoid the new flag for now since mavctap uses real NICs as low= er > >> > device? > >> > > >> > Is there any other restriction besides requiring driver to not r= ecycle > >> > the skb? Are there any drivers that recycle TX skbs? > > > > Not just recycling skbs, keeping reference to any of the pages in t= he > > skb. Another requirement is to invoke the callback > > in a timely fashion. =C2=A0For 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? >=20 > 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 th= e > 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() l= ooks > >> 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? >=20 > It's making copy of the skb_shinfo earlier, so the pages refcount > stays the same. >=20 > Best Regards, > Micha=C5=82 Miros=C5=82aw 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. --=20 MST