From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] Add a page cache-backed balloon device driver. Date: Mon, 03 Sep 2012 08:35:27 +0200 Message-ID: <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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <87obnzxaxv.fsf@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: Frank Swiderski , Andrea Arcangeli , Rik van Riel , Rafael Aquini , kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, mikew@google.com, Ying Han , virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org 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. Paolo