From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756718Ab0DNUkR (ORCPT ); Wed, 14 Apr 2010 16:40:17 -0400 Received: from moutng.kundenserver.de ([212.227.126.186]:63213 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756537Ab0DNUkP (ORCPT ); Wed, 14 Apr 2010 16:40:15 -0400 From: Arnd Bergmann To: "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 22:39:49 +0200 User-Agent: KMail/1.13.2 (Linux/2.6.31-19-generic; KDE/4.4.2; x86_64; ; ) 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 References: <1270805865-16901-1-git-send-email-xiaohui.xin@intel.com> <201004141835.57673.arnd@arndb.de> <20100414203142.GA11321@redhat.com> In-Reply-To: <20100414203142.GA11321@redhat.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201004142239.50299.arnd@arndb.de> X-Provags-ID: V01U2FsdGVkX19VGRmB/zsKlN+JnVswTgT1Uq8i+4RPSkyVyDB WSmtBBurKhFjxCWiND0NgupikNTCZNoxXPxFf2vhkaOh6vfYVo JGGMUwSeucBRqDbP70PCw== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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