From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754721Ab1KDJs0 (ORCPT ); Fri, 4 Nov 2011 05:48:26 -0400 Received: from ozlabs.org ([203.10.76.45]:54920 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080Ab1KDJsY (ORCPT ); Fri, 4 Nov 2011 05:48:24 -0400 From: Rusty Russell To: Sasha Levin Cc: "Michael S. Tsirkin" , linux-kernel , kvm , virtualization , Anthony Liguori Subject: Re: virtio-pci new configuration proposal In-Reply-To: <1320309203.29407.1.camel@lappy> References: <1320259767.22582.2.camel@lappy> <8762j2t19l.fsf@rustcorp.com.au> <1320309203.29407.1.camel@lappy> User-Agent: Notmuch/0.6.1-1 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) Date: Fri, 04 Nov 2011 20:14:43 +1030 Message-ID: <87lirwrzlg.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 03 Nov 2011 10:33:23 +0200, Sasha Levin wrote: > On Thu, 2011-11-03 at 12:28 +1030, Rusty Russell wrote: > > 2) I don't think we want to turn the device-specific config into a > > linked list. We haven't needed variable-length config (yet!), and > > it's (slightly) more complex. That's also the part of the spec which > > is shared with non-PCI virtio implementations. > > Variable length config wasn't used yet because space in the device > specific space was reserved for a feature even if that feature wasn't > used. > > For example, the MAC feature reserved 6 bytes in the config space for > the MAC even if VIRTIO_NET_F_MAC wasn't enabled. Here we can just avoid > having it pollute the config space until it's enabled. Exactly. But we haven't had a problem so far; but we don't put arbitrarily large fields in there. > I don't think it'll have any impact on non-PCI implementations since the > "pointers" are simply offsets from the beginning of the config space, > and are not PCI specific in any way. But the drivers currently just use offsetof() to access it, eg: if (virtio_config_val_len(vdev, VIRTIO_NET_F_MAC, offsetof(struct virtio_net_config, mac), dev->dev_addr, dev->addr_len) < 0) That would have to change, and that means a change for drivers and for the non-PCI implementations. Hence I think this is a step too far. > > 3) If we're changing the queue layout, it's a chance to fix a > > longstanding bug: let the guest notify the host of preferred > > queue size and alignment. > > Yup, we can do that. The seabios guys will definitely thank you! Cheers, Rusty.