From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1LMjdF-0005ll-88 for qemu-devel@nongnu.org; Tue, 13 Jan 2009 08:48:37 -0500 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1LMjdD-0005i3-9z for qemu-devel@nongnu.org; Tue, 13 Jan 2009 08:48:36 -0500 Received: from [199.232.76.173] (port=47275 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1LMjdD-0005hm-4h for qemu-devel@nongnu.org; Tue, 13 Jan 2009 08:48:35 -0500 Received: from mx2.redhat.com ([66.187.237.31]:44226) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1LMjdC-0007MW-L8 for qemu-devel@nongnu.org; Tue, 13 Jan 2009 08:48:34 -0500 Subject: Re: [Qemu-devel] virtio breakage with 2.6.25 guest kernel From: Mark McLoughlin In-Reply-To: <496C9813.7000609@suse.de> References: <496C9813.7000609@suse.de> Content-Type: text/plain Date: Tue, 13 Jan 2009 13:48:28 +0000 Message-Id: <1231854508.4290.186.camel@localhost.localdomain> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Reply-To: Mark McLoughlin , qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: rusty@rustcorp.com.au Hi Alex, (Dropping kvm-devel@sf.net - it's kvm@vger.kernel.org now anyway) On Tue, 2009-01-13 at 14:33 +0100, Alexander Graf wrote: > while I don't fully understand the problem, here's what I experience so far: > > When using an openSUSE 11.0 kernel (2.6.25) in the guest, virtio on tap > breaks with current KVM git, while it used to work before (haven't > bisected, definitely worked in kvm-78, but is probably due to Anthony's > rewrite). It shows the following message (comes from qemu): > > virtio-net header not in first element > > This is because qemu expects a message with mergeable rx bufs (12 > bytes), but if I see things correctly the old kernel doesn't support > that feature (sends 10 bytes). I put in some debug messages on IO > reads/writes in the qemu virtio handler and got these: > > virtio IO read: 0 = 0x100bba3 > virtio IO write: 0x4 = 0x100bba3 > virtio: setting features 0x100bba3 > > So I believe the feature masking is somewhat non-functional? Or did I > read the masking code in qemu's virtio-net wrong? > > Basically for me, current KVM git broke old guest support, which is a > clear regression and should not happen. Does anybody with more expertise > in virtio want to take on this? Yeah, I noticed this one last week. The issue here is that 2.6.25 virtio erroneously says it supports all features supported by the host - i.e. it's a guest kernel bug. With recent qemu, that causes us to enable mergeable receive buffers and the guest barfs. See: mask = ioread32(vp_dev->ioaddr + VIRTIO_PCI_HOST_FEATURES); if (mask & (1 << bit)) { mask |= (1 << bit); iowrite32(mask, vp_dev->ioaddr + VIRTIO_PCI_GUEST_FEATURES); This got fixed in 2.6.26 by: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c45a6816 I did consider forwarding this to stable@vger.kernel.org, but apparently the 2.6.25.x series is finished now anyway. Cheers, Mark.