From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1LzZJz-0007UM-C9 for qemu-devel@nongnu.org; Thu, 30 Apr 2009 12:41:15 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1LzZJu-0007RD-Jq for qemu-devel@nongnu.org; Thu, 30 Apr 2009 12:41:14 -0400 Received: from [199.232.76.173] (port=51925 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1LzZJu-0007R1-D3 for qemu-devel@nongnu.org; Thu, 30 Apr 2009 12:41:10 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:38989) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1LzZJt-0000UP-UM for qemu-devel@nongnu.org; Thu, 30 Apr 2009 12:41:10 -0400 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e9.ny.us.ibm.com (8.13.1/8.13.1) with ESMTP id n3UGUI4G005788 for ; Thu, 30 Apr 2009 12:30:18 -0400 Received: from d01av02.pok.ibm.com (d01av02.pok.ibm.com [9.56.224.216]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v9.2) with ESMTP id n3UGf8p8054342 for ; Thu, 30 Apr 2009 12:41:08 -0400 Received: from d01av02.pok.ibm.com (loopback [127.0.0.1]) by d01av02.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id n3UGdIeg003361 for ; Thu, 30 Apr 2009 12:39:18 -0400 Message-ID: <49F9D4A1.7070005@us.ibm.com> Date: Thu, 30 Apr 2009 11:41:05 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH 9/9] Introduce VLANClientState::cleanup() References: <1239812969-8320-2-git-send-email-markmc@redhat.com> <200904301702.32282.paul@codesourcery.com> <49F9CFCA.8000002@redhat.com> <200904301731.04902.paul@codesourcery.com> <49F9D2BB.2060608@redhat.com> <49F9D3D8.3000704@us.ibm.com> In-Reply-To: <49F9D3D8.3000704@us.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Mark McLoughlin , Marcelo Tosatti , Paul Brook , Markus Armbruster , qemu-devel@nongnu.org Anthony Liguori wrote: > Avi Kivity wrote: >> True, but is it actually useful? >> >> Short-circuiting two guest devices probably isn't. Adding host >> devices (like recording packets) might be. > > But you can do this by just inserting yourself between the > frontend/backend. You can do this dynamically as long as you don't > require feature negotiation. Dynamic negotiation is prohibitively > difficult and one of the primary reasons we haven't been able to merge > vnet support for tap yet. Paul: in KVM, tap devices support optional GSO/checksum offload support. This is enabled for the tap device depending on whether you're using virtio-net and your host kernel supports it. In KVM, having > 1 guest device on a vlan is broken when a tap device in on a vlan. This is by design because the dynamic feature negotiation isn't always possible. The other option would be to disallow additional VLAN that can't support the same features or do full GSO emulation within QEMU. Neither of these are very good options. No one has complained yet to the best of my knowledge that you can't have more than one guest device on a vlan in KVM. This is simply because no one ever does it :-) -- Regards, Anthony Liguori