From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works Date: Mon, 10 Sep 2012 08:38:09 +0200 Message-ID: <504D8AD1.6050006@redhat.com> References: <20120907105335.GB17211@redhat.com> <5049D899.60705@redhat.com> <20120907121712.GA17397@redhat.com> <5049E717.8080307@redhat.com> <20120907124432.GB17397@redhat.com> <5049FEDD.40303@redhat.com> <20120907142545.GC17397@redhat.com> <504A0858.4080508@redhat.com> <20120908222221.GA20588@redhat.com> <504D7F95.9070700@redhat.com> <20120910060359.GB16819@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120910060359.GB16819@redhat.com> 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: "Michael S. Tsirkin" Cc: fes@google.com, aarcange@redhat.com, riel@redhat.com, kvm@vger.kernel.org, yvugenfi@redhat.com, linux-kernel@vger.kernel.org, mikew@google.com, yinghan@google.com, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org Il 10/09/2012 08:03, Michael S. Tsirkin ha scritto: > On Mon, Sep 10, 2012 at 07:50:13AM +0200, Paolo Bonzini wrote: >> Il 09/09/2012 00:22, Michael S. Tsirkin ha scritto: >>>> Almost. One is "the guest, if really needed, can tell the host of >>>> pages". If not negotiated, and the host does not support it, the host >>>> must break the guest (e.g. fail to offer any virtqueues). >>> >>> There is no way in spec to break the guest. >>> You can not fail to offer virtqueues. >> >> You can always return 0 for the first queue. > > I don't think guest drivers recover gracefully from this. > Do they? No, that's the point ("break the guest" is really "break the driver"). >>> Besides, there is no guarantee that virtqueue setup >>> happens after feature negotiation. >> >> It is the only way that makes sense though (unless the guest would write >> 0 for its features). Should we change that? > > I do think it would be nice to add a generic way for device to > notify guest about an internal failure. > This can only happen after DRIVER_OK status is written though, > and since existing drivers do not expect such failure, it might > be too late. Agreed. >>>> The other is "the guest, though, would prefer not to do so". It is >>>> different because the guest can proceed in a fallback mode even if the >>>> host doesn't offer it. >>> >>> I think I get what your proposed SILENT means what I do not get >>> is the motivation. It looks like a premature optimization to me. >> >> The motivation is to let the driver choose between two behaviors: the >> current one where ballooning is only done on request, and a more >> aggressive one. > > Yes but why is being silent any good? Optimization? > Any data to show that it will help some workload? Idle guests can move cache pages to the balloon. You can overcommit more aggressively, because the host can madvise away a lot more memory. >>> OK so TELL says *when* to notify host, SILENT if set allows guest >>> to skip leak notifications? In this case TELL should just be ignored >>> when SILENT is set. >> >> Yeah, that was my first idea. However, there are existing drivers that >> ignore SILENT, so that would not be 100% exact. > > Not sure I follow the logic. > They don't ack SILENT so that would be 100% exact. Hmm, then I'm not sure I follow yours. We agreed that delaying notifications or skipping them is really the same thing, right? I think we're just stuck in a linguistic problem, with "must not" being wrong and "does not have to" being too verbose. Calling it VIRTIO_BALLOON_F_SILENT_DEFLATE was a workaround for this, but perhaps it adds more confusion. Paolo