linux-kernel - RE: [PATCH v9 5/5] virtio-balloon: VIRTIO_BALLOON_F_MISC

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Date:   Sun, 7 May 2017 04:20:51 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "david@...hat.com" <david@...hat.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "liliang.opensource@...il.com" <liliang.opensource@...il.com>
Subject: RE: [PATCH v9 5/5] virtio-balloon: VIRTIO_BALLOON_F_MISC_VQ

On 05/06/2017 06:21 AM, Michael S. Tsirkin wrote:
> On Thu, Apr 27, 2017 at 02:33:44PM +0800, Wei Wang wrote:
> > On 04/14/2017 01:08 AM, Michael S. Tsirkin wrote:
> > > On Thu, Apr 13, 2017 at 05:35:08PM +0800, Wei Wang wrote:
> > > > Add a new vq, miscq, to handle miscellaneous requests between the
> > > > device and the driver.
> > > >
> > > > This patch implemnts the
> VIRTIO_BALLOON_MISCQ_INQUIRE_UNUSED_PAGES
> > > implements
> > >
> > > > request sent from the device.
> > > Commands are sent from host and handled on guest.
> > > In fact how is this so different from stats?
> > > How about reusing the stats vq then? You can use one buffer for
> > > stats and one buffer for commands.
> > >
> >
> > The meaning of the two vqs is a little different. statq is used for
> > reporting statistics, while miscq is intended to be used to handle
> > miscellaneous requests from the guest or host
> 
> misc just means "anything goes". If you want it to mean "commands" name it so.

Ok, will change it.

> > (I think it can
> > also be used the other way around in the future when other new
> > features are added which need the guest to send requests and the host
> > to provide responses).
> >
> > I would prefer to have them separate, because:
> > If we plan to combine them, we need to put the previous statq related
> > implementation under miscq with a new command (I think we can't
> > combine them without using commands to distinguish the two features).
> 
> Right.

> > In this way, an old driver won't work with a new QEMU or a new driver
> > won't work with an old QEMU. Would this be considered as an issue
> > here?
> 
> Compatibility is and should always be handled using feature flags.  There's a
> feature flag for this, isn't it?

The negotiation of the existing feature flag, VIRTIO_BALLOON_F_STATS_VQ
only indicates the support of the old statq implementation. To move the statq
implementation under cmdq, I think we would need a new feature flag for the
new statq implementation:
#define VIRTIO_BALLOON_F_CMDQ_STATS      5

What do you think?

Best,
Wei