linux-kernel - Re: [PATCH 2/2] virtio: document virtio_config

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Message-ID: <20190104133939.7d52ebb3.cohuck@redhat.com>
Date:   Fri, 4 Jan 2019 13:39:39 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, virtio-dev@...ts.oasis-open.org,
        Wei Wang <wei.w.wang@...el.com>,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH 2/2] virtio: document virtio_config_ops restrictions

On Thu, 3 Jan 2019 11:28:28 -0500
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Thu, Jan 03, 2019 at 05:08:04PM +0100, Cornelia Huck wrote:
> > Some transports (e.g. virtio-ccw) implement virtio operations that
> > seem to be a simple read/write as something more involved that
> > cannot be done from an atomic context.
> > 
> > Give at least a hint about that.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@...hat.com>
> > ---
> >  include/linux/virtio_config.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > index 7087ef946ba7..987b6491b946 100644
> > --- a/include/linux/virtio_config.h
> > +++ b/include/linux/virtio_config.h
> > @@ -12,6 +12,11 @@ struct irq_affinity;
> >  
> >  /**
> >   * virtio_config_ops - operations for configuring a virtio device
> > + * Note: Do not assume that a transport implements all of the operations
> > + *       getting/setting a value as a simple read/write! Generally speaking,
> > + *       any of @get/@set, @get_status/@..._status, or @get_features/
> > + *       @finalize_features are NOT safe to be called from an atomic
> > + *       context.
> >   * @get: read the value of a configuration field
> >   *	vdev: the virtio_device
> >   *	offset: the offset of the configuration field  
> 
> Then might_sleep in virtio_cread/virtio_cwrite and
> friends would be appropriate? I guess we'll need to fix
> balloon first.

Yes, it makes sense to go over the code and add might_sleep to
functions where it makes sense after the balloon changes have been
merged.