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 19:50:21 +0300 Message-ID: <20110518165021.GC22001@redhat.com> References: <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=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: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, May 18, 2011 at 01:40:29PM +0200, Micha=C5=82 Miros=C5=82aw wro= te: > W dniu 18 maja 2011 13:17 u=C5=BCytkownik Michael S. Tsirkin > napisa=C5=82: > > On Wed, May 18, 2011 at 01:10:50PM +0200, Micha=C5=82 Miros=C5=82aw= 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=C5=82 Miros=C5=82aw wr= ote: > >> >> > 2011/5/17 Shirley Ma : > >> >> > > 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 no= t recycle > >> >> > the skb? Are there any drivers that recycle TX skbs? > >> > > >> > Not just recycling skbs, keeping reference to any of the pages i= n the > >> > 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? > >> > >> Hmm. But every hardware driver supporting SG will keep reference t= o > >> 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 reaso= n. > > > > 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= the > >> pages are released? > > No one should reference the pages after the callback is invoked, ye= s. >=20 > >> >> Not more other restrictions, skb clone is OK. pskb_expand_head(= ) looks > >> >> 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? > >> It's making copy of the skb_shinfo earlier, so the pages refcount > >> stays the same. > > 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 > Hmm. I tool a quick look at skb_clone(), and it looks like this > sequence will break this scheme: >=20 > skb2 =3D skb_clone(skb...); > kfree_skb(skb) or pskb_expand_head(skb); /* callback called */ > [use skb2, pages still referenced] > kfree_skb(skb); /* callback called again */ > This sequence is common in bridge, might be in other places. >=20 > Maybe this ubuf thing should just track clones? This will make it wor= k > on all devices then. >=20 > Best Regards, > Micha=C5=82 Miros=C5=82aw Well bridge has the problem that packet might get anywhere and it's really hard to track. Same for tun - it can get queued forever. veth, loopback are all a problem I think. IOW we really want to limit this to real physical NICs which mostly all DTRT. Whitelisting them with a new flag is likely the most concervative approach, no? --=20 MST