Re: [PATCH 2/4] vhost-vdpa: Introduce RESUME backend feature bit

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 14, 2022 at 02:09:02PM +0800, Jason Wang wrote:
> On Fri, Oct 14, 2022 at 2:05 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Oct 14, 2022 at 01:58:38PM +0800, Jason Wang wrote:
> > > On Thu, Oct 13, 2022 at 11:35 PM <sebastien.boeuf@intel.com> wrote:
> > > >
> > > > From: Sebastien Boeuf <sebastien.boeuf@intel.com>
> > > >
> > > > Userspace knows if the device can be resumed or not by checking this
> > > > feature bit.
> > > >
> > > > It's only exposed if the vdpa driver backend implements the resume()
> > > > operation callback. Userspace trying to negotiate this feature when it
> > > > hasn't been exposed will result in an error.
> > > >
> > > > Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
> > > > ---
> > > >  drivers/vhost/vdpa.c             | 19 ++++++++++++++++++-
> > > >  include/uapi/linux/vhost_types.h |  2 ++
> > > >  2 files changed, 20 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > > > index 166044642fd5..161727e1a9a5 100644
> > > > --- a/drivers/vhost/vdpa.c
> > > > +++ b/drivers/vhost/vdpa.c
> > > > @@ -355,6 +355,14 @@ static bool vhost_vdpa_can_suspend(const struct vhost_vdpa *v)
> > > >         return ops->suspend;
> > > >  }
> > > >
> > > > +static bool vhost_vdpa_can_resume(const struct vhost_vdpa *v)
> > > > +{
> > > > +       struct vdpa_device *vdpa = v->vdpa;
> > > > +       const struct vdpa_config_ops *ops = vdpa->config;
> > > > +
> > > > +       return ops->resume;
> > > > +}
> > > > +
> > > >  static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
> > > >  {
> > > >         struct vdpa_device *vdpa = v->vdpa;
> > > > @@ -602,11 +610,18 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> > > >                 if (copy_from_user(&features, featurep, sizeof(features)))
> > > >                         return -EFAULT;
> > > >                 if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
> > > > -                                BIT_ULL(VHOST_BACKEND_F_SUSPEND)))
> > > > +                                BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
> > > > +                                BIT_ULL(VHOST_BACKEND_F_RESUME)))
> > > >                         return -EOPNOTSUPP;
> > > >                 if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
> > > >                      !vhost_vdpa_can_suspend(v))
> > > >                         return -EOPNOTSUPP;
> > > > +               if ((features & BIT_ULL(VHOST_BACKEND_F_RESUME)) &&
> > > > +                    !vhost_vdpa_can_resume(v))
> > > > +                       return -EOPNOTSUPP;
> > > > +               if (!(features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
> > > > +                    (features & BIT_ULL(VHOST_BACKEND_F_RESUME)))
> > > > +                       return -EINVAL;
> > >
> > > Is it better to do the check during the probe? It should be a bug that
> > > we're having a parent that can only do resume but not suspend.
> > >
> > > Thanks
> >
> > well we separated them in the spec ...
> > suspend could have other uses, I won't say it's an invalid
> > config.
> 
> For suspend only, yes. But we should fail the probe with a resume
> only, this is somehow the above code wants to check. Or anything I
> missed?
> 
> Thanks

I am not sure but I would say failing probe is a drastic measure.
if we have no use for a given combination of features let us clear the
feature bit in validation.

> >
> > > >                 vhost_set_backend_features(&v->vdev, features);
> > > >                 return 0;
> > > >         }
> > > > @@ -658,6 +673,8 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> > > >                 features = VHOST_VDPA_BACKEND_FEATURES;
> > > >                 if (vhost_vdpa_can_suspend(v))
> > > >                         features |= BIT_ULL(VHOST_BACKEND_F_SUSPEND);
> > > > +               if (vhost_vdpa_can_resume(v))
> > > > +                       features |= BIT_ULL(VHOST_BACKEND_F_RESUME);
> > > >                 if (copy_to_user(featurep, &features, sizeof(features)))
> > > >                         r = -EFAULT;
> > > >                 break;
> > > > diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
> > > > index 53601ce2c20a..c5690a8992d8 100644
> > > > --- a/include/uapi/linux/vhost_types.h
> > > > +++ b/include/uapi/linux/vhost_types.h
> > > > @@ -163,5 +163,7 @@ struct vhost_vdpa_iova_range {
> > > >  #define VHOST_BACKEND_F_IOTLB_ASID  0x3
> > > >  /* Device can be suspended */
> > > >  #define VHOST_BACKEND_F_SUSPEND  0x4
> > > > +/* Device can be resumed */
> > > > +#define VHOST_BACKEND_F_RESUME  0x5
> > > >
> > > >  #endif
> > > > --
> > > > 2.34.1
> > > >
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