From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC][PATCH v3 1/3] A device for zero-copy based on KVM virtio-net. Date: Wed, 14 Apr 2010 23:40:03 +0300 Message-ID: <20100414204003.GC11321@redhat.com> References: <1270805865-16901-1-git-send-email-xiaohui.xin@intel.com> <201004141835.57673.arnd@arndb.de> <20100414203142.GA11321@redhat.com> <201004142239.50299.arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: xiaohui.xin@intel.com, netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu, davem@davemloft.net, jdike@linux.intel.com To: Arnd Bergmann Return-path: Content-Disposition: inline In-Reply-To: <201004142239.50299.arnd@arndb.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Apr 14, 2010 at 10:39:49PM +0200, Arnd Bergmann wrote: > On Wednesday 14 April 2010 22:31:42 Michael S. Tsirkin wrote: > > On Wed, Apr 14, 2010 at 06:35:57PM +0200, Arnd Bergmann wrote: > > > On Wednesday 14 April 2010, Michael S. Tsirkin wrote: > > > > > > > > > > > > qemu needs the ability to inject raw packets into device > > > > > > from userspace, bypassing vhost/virtio (for live migration). > > > > > > > > > > Ok, but since there is only a write callback and no read, it won't > > > > > actually be able to do this with the current code, right? > > > > > > > > I think it'll work as is, with vhost qemu only ever writes, > > > > never reads from device. We'll also never need GSO etc > > > > which is a large part of what tap does (and macvtap will > > > > have to do). > > > > > > Ah, I see. I didn't realize that qemu needs to write to the > > > device even if vhost is used. But for the case of migration to > > > another machine without vhost, wouldn't qemu also need to read? > > > > Not that I know. Why? > > Well, if the guest not only wants to send data but also receive > frames coming from other machines, they need to get from the kernel > into qemu, and the only way I can see for doing that is to read > from this device if there is no vhost support around on the new > machine. > > Maybe we're talking about different things here. > > Arnd mpassthrough is currently useless without vhost. If the new machine has no vhost, it can't use mpassthrough :) -- MST