Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933120Ab1ERQui (ORCPT ); Wed, 18 May 2011 12:50:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:10220 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932859Ab1ERQuh (ORCPT ); Wed, 18 May 2011 12:50:37 -0400 Date: Wed, 18 May 2011 19:50:21 +0300 From: "Michael S. Tsirkin" To: =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= 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 Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice 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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3429 Lines: 77 On Wed, May 18, 2011 at 01:40:29PM +0200, Michał Mirosław wrote: > W dniu 18 maja 2011 13:17 użytkownik Michael S. Tsirkin > napisał: > > On Wed, May 18, 2011 at 01:10:50PM +0200, Michał Mirosław 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, Michał Mirosław wrote: > >> >> > 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 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. > > > >> 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, yes. > > >> >> 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. > > Hmm. I tool a quick look at skb_clone(), and it looks like this > sequence will break this scheme: > > skb2 = 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. > > Maybe this ubuf thing should just track clones? This will make it work > on all devices then. > > Best Regards, > Michał Mirosław 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? -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/