From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759463Ab2IFDDT (ORCPT ); Wed, 5 Sep 2012 23:03:19 -0400 Received: from ozlabs.org ([203.10.76.45]:40604 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754286Ab2IFDAx (ORCPT ); Wed, 5 Sep 2012 23:00:53 -0400 From: Rusty Russell To: Paolo Bonzini Cc: Frank Swiderski , "Michael S. Tsirkin" , Andrea Arcangeli , Rik van Riel , Rafael Aquini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mikew@google.com, Ying Han , virtualization@lists.linux-foundation.org Subject: Re: [PATCH] Add a page cache-backed balloon device driver. In-Reply-To: <50444FAF.7030304@redhat.com> References: <1340742778-11282-1-git-send-email-fes@google.com> <4FEA1E2E.4020806@redhat.com> <20120626214709.GA15406@redhat.com> <87obnzxaxv.fsf@rustcorp.com.au> <50444FAF.7030304@redhat.com> User-Agent: Notmuch/0.13.2 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) Date: Thu, 06 Sep 2012 11:05:12 +0930 Message-ID: <87sjawhr1r.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: > Il 02/07/2012 02:29, Rusty Russell ha scritto: >> VIRTIO_BALLOON_F_MUST_TELL_HOST >> implies you should tell the host (eventually). I don't know if any >> implementations actually care though. > > This is indeed broken, because it is a "negative" feature: it tells you > that "implicit deflate" is _not_ supported. > > Right now, QEMU refuses migration if the target does not support all the > features that were negotiated. But then: > > - a migration from non-MUST_TELL_HOST to MUST_TELL_HOST will succeed, > which is wrong; > > - a migration from MUST_TELL_HOST to non-MUST_TELL_HOST will fail, which > is useless. > >> We could add a VIRTIO_BALLOON_F_NEVER_TELL_DEFLATE which would mean the >> deflate vq need not be used at all. > > That would work. At the same time we could deprecate MUST_TELL_HOST. > Certainly the guest implementations don't care, or we would have > experienced problems such as the one above. The QEMU implementation > also does not care but, for example, a Xen implementation would care. OK; I'm not sure we need to deprecate MUST_TELL_HOST, though since it's never actually been used there's a good argument. VIRTIO_BALLOON_F_SILENT_DEFLATE (or whatever it's called) would obviously mean you couldn't ack VIRTIO_BALLOON_F_MUST_TELL_HOST. Patches welcome! Cheers, Rusty.