From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932261Ab2IGADS (ORCPT ); Thu, 6 Sep 2012 20:03:18 -0400 Received: from ozlabs.org ([203.10.76.45]:38763 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755234Ab2IGAC0 (ORCPT ); Thu, 6 Sep 2012 20:02:26 -0400 From: Rusty Russell To: Paolo Bonzini , "Michael S. Tsirkin" Cc: fes@google.com, aarcange@redhat.com, riel@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mikew@google.com, yinghan@google.com, virtualization@lists.linux-foundation.org Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works In-Reply-To: <5048935A.8090308@redhat.com> References: <1346917610-14568-1-git-send-email-pbonzini@redhat.com> <20120906084736.GF17656@redhat.com> <50486BB2.7070108@redhat.com> <20120906094442.GA22816@redhat.com> <50487382.8030303@redhat.com> <20120906105301.GC32325@redhat.com> <5048935A.8090308@redhat.com> User-Agent: Notmuch/0.13.2 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) Date: Fri, 07 Sep 2012 09:15:46 +0930 Message-ID: <87wr06hg0l.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 Paolo Bonzini writes: > Instead, VIRTIO_NET_F_HOST_MUST_SEND_GARP would be a "negative" feature: > if set, the host _may not_ rely on the guest to send a gARP. Similarly > if VIRTIO_BALLOON_F_MUST_TELL_HOST is set, the guest _may not_ use > ballooned pages directly. > > There are _no_ other negative features besides > VIRTIO_BALLOON_F_MUST_TELL_HOST in the spec, and for a good > reason---because they're broken. > > (Hmm, actually we have one, VIRTIO_BLK_F_RO. It is also a bit broken, > but it is not so important because it depends on user input more than > hypervisor version). Yes, this is the key observation, and an important lesson for the future. Thanks! Note that these two negative features were in the original spec, where it's assumed that every device supports them. That's not explicitly documented, however. I like killing the totally unused feature. Cheers, Rusty.