From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel Date: Tue, 14 Sep 2010 21:01:56 +0200 Message-ID: <20100914190156.GA16037@redhat.com> References: <1284410580.13351.10.camel@localhost.localdomain> <4C8F3C77.7010302@redhat.com> <1284476719.13351.35.camel@localhost.localdomain> <201009141721.13202.arnd@arndb.de> <20100914152231.GA13105@redhat.com> <1284480025.13351.49.camel@localhost.localdomain> <20100914162952.GB13560@redhat.com> <1284483745.13351.71.camel@localhost.localdomain> <20100914182707.GB15549@redhat.com> <1284490143.13351.82.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Arnd Bergmann , Avi Kivity , "Xin, Xiaohui" , David Miller , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Shirley Ma Return-path: Content-Disposition: inline In-Reply-To: <1284490143.13351.82.camel@localhost.localdomain> Sender: kvm-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, Sep 14, 2010 at 11:49:03AM -0700, Shirley Ma wrote: > On Tue, 2010-09-14 at 20:27 +0200, Michael S. Tsirkin wrote: > > As others said, the harder issues for TX are in determining that it's > > safe > > to unpin the memory, and how much memory is it safe to pin to beging > > with. For RX we have some more complexity. > > I think unpin the memory is in kfree_skb() whenever the last reference > is gone for TX. What we discussed about here is when/how vhost get > notified to update ring buffer descriptors. Do I misunderstand something > here? Right, that's a better way to put it. > > Well it's up to you of course, but this is not what I would try: > > if you look at the > > patchset vhost changes is not the largest part of it, > > so this sounds a bit like effort duplication. > > > > TX only is also much less interesting than full zero copy. > > It's not true. From the performance, TX only has gained big improvement. > We need to identify how much gain from TX zero copy, and how much gain > from RX zero copy. I was speaking from the code point of view: since we'll want both TX and RX eventually it's nice to see that some thought was given to RX even if we only merge TX as a first step. >>From the product POV, RX is already slower (more interrupts, etc) than TX so speeding it up might be more important, but I agree, every bit helps. > > I think that you should be able to simply combine > > the two drivers together, add an ioctl to > > enable/disable zero copy mode of operation. > > That could work. But what's the purpose to have two drivers if one > driver can handle it? > > Thanks > Shirley This was just an idea: I thought it's a good way for people interested in this zero copy thing to combine forces and avoid making the same mistakes, but it's not a must of course. -- MST