Linux Virtualization - Sep 2021 - [PATCH v13 05/13] vdpa: Add reset callback in vdpa_config_ops

On Mon, Sep 06, 2021 at 02:09:25PM +0800, Yongji Xie
wrote:
> On Mon, Sep 6, 2021 at 1:56 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
> >
> > On Tue, Aug 31, 2021 at 06:36:26PM +0800, Xie Yongji wrote:
> > > This adds a new callback to support device specific reset
> > > behavior. The vdpa bus driver will call the reset function
> > > instead of setting status to zero during resetting.
> > >
> > > Signed-off-by: Xie Yongji <xieyongji at bytedance.com>
> >
> >
> > This does gloss over a significant change though:
> >
> >
> > > ---
> > > @@ -348,12 +352,12 @@ static inline struct device
*vdpa_get_dma_dev(struct vdpa_device *vdev)
> > >       return vdev->dma_dev;
> > >  }
> > >
> > > -static inline void vdpa_reset(struct vdpa_device *vdev)
> > > +static inline int vdpa_reset(struct vdpa_device *vdev)
> > >  {
> > >       const struct vdpa_config_ops *ops = vdev->config;
> > >
> > >       vdev->features_valid = false;
> > > -     ops->set_status(vdev, 0);
> > > +     return ops->reset(vdev);
> > >  }
> > >
> > >  static inline int vdpa_set_features(struct vdpa_device *vdev,
u64 features)
> >
> >
> > Unfortunately this breaks virtio_vdpa:
> >
> >
> > static void virtio_vdpa_reset(struct virtio_device *vdev)
> > {
> >         struct vdpa_device *vdpa = vd_get_vdpa(vdev);
> >
> >         vdpa_reset(vdpa);
> > }
> >
> >
> > and there's no easy way to fix this, kernel can't recover
> > from a reset failure e.g. during driver unbind.
> >
> 
> Yes, but it should be safe with the protection of software IOTLB even
> if the reset() fails during driver unbind.
> 
> Thanks,
> Yongji
Hmm. I don't see it.
What exactly will happen? What prevents device from poking at
memory after reset? Note that dma unmap in e.g. del_vqs happens
too late.  And what about e.g. interrupts?
E.g. we have this:

        /* Virtqueues are stopped, nothing can use vblk->vdev anymore. */
        vblk->vdev = NULL;

and this is no longer true at this point.


-- 
MST