linux-kernel - Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

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Message-ID: <20240710021441-mutt-send-email-mst@kernel.org>
Date: Wed, 10 Jul 2024 02:16:04 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	virtualization@...ts.linux.dev
Subject: Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

On Wed, Jul 10, 2024 at 11:23:20AM +0800, Jason Wang wrote:
> On Fri, Jul 5, 2024 at 6:09 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > QEMU implemented the configuration
> >         VIRTIO_BALLOON_F_REPORTING && ! VIRTIO_BALLOON_F_FREE_PAGE_HINT
> > incorrectly: it then uses vq3 for reporting, spec says it is always 4.
> >
> > This is masked by a corresponding bug in driver:
> > add a work around as I'm going to try and fix the driver bug.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  drivers/virtio/virtio_balloon.c | 19 +++++++++++++++++--
> >  1 file changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 9a61febbd2f7..7dc3fcd56238 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -597,8 +597,23 @@ static int init_vqs(struct virtio_balloon *vb)
> >
> >         err = virtio_find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX, vqs,
> >                               callbacks, names, NULL);
> > -       if (err)
> > -               return err;
> > +       if (err) {
> > +               /*
> > +                * Try to work around QEMU bug which since 2020 confused vq numbers
> > +                * when VIRTIO_BALLOON_F_REPORTING but not
> > +                * VIRTIO_BALLOON_F_FREE_PAGE_HINT are offered.
> > +                */
> > +               if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
> > +                   !virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> > +                       names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "reporting_vq";
> > +                       callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = balloon_ack;
> > +                       err = virtio_find_vqs(vb->vdev,
> > +                                             VIRTIO_BALLOON_VQ_REPORTING, vqs, callbacks, names, NULL);
> > +               }
> > +
> > +               if (err)
> > +                       return err;
> > +       }
> >
> >         vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> >         vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> > --
> > MST
> >
> 
> Acked-by: Jason Wang <jasowang@...hat.com>
> 
> Do we need a spec to say this is something that needs to be considered
> by the driver?
> 
> Thanks

I'd say it's a temporary situation that we won't need to bother
about in several years.

-- 
MST