RE: [PATCH v4 3/4] vhost-vdpa: uAPI to stop the device

[Parav Pandit]

> From: Eugenio Perez Martin <eperezma@xxxxxxxxxx>
> Sent: Wednesday, June 1, 2022 7:15 AM
> 
> On Wed, Jun 1, 2022 at 1:03 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > On Thu, May 26, 2022 at 02:43:37PM +0200, Eugenio Pérez wrote:
> > > The ioctl adds support for stop the device from userspace.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
> > > ---
> > >  drivers/vhost/vdpa.c       | 18 ++++++++++++++++++
> > >  include/uapi/linux/vhost.h | 14 ++++++++++++++
> > >  2 files changed, 32 insertions(+)
> > >
> > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index
> > > 32713db5831d..d1d19555c4b7 100644
> > > --- a/drivers/vhost/vdpa.c
> > > +++ b/drivers/vhost/vdpa.c
> > > @@ -478,6 +478,21 @@ static long vhost_vdpa_get_vqs_count(struct
> vhost_vdpa *v, u32 __user *argp)
> > >       return 0;
> > >  }
> > >
> > > +static long vhost_vdpa_stop(struct vhost_vdpa *v, u32 __user *argp)
> > > +{
> > > +     struct vdpa_device *vdpa = v->vdpa;
> > > +     const struct vdpa_config_ops *ops = vdpa->config;
> > > +     int stop;
> > > +
> > > +     if (!ops->stop)
> > > +             return -EOPNOTSUPP;
> > > +
> > > +     if (copy_from_user(&stop, argp, sizeof(stop)))
> > > +             return -EFAULT;
> > > +
> > > +     return ops->stop(vdpa, stop);
> > > +}
> > > +
> > >  static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int
> cmd,
> > >                                  void __user *argp)  { @@ -650,6
> > > +665,9 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> > >       case VHOST_VDPA_GET_VQS_COUNT:
> > >               r = vhost_vdpa_get_vqs_count(v, argp);
> > >               break;
> > > +     case VHOST_VDPA_STOP:
> > > +             r = vhost_vdpa_stop(v, argp);
> > > +             break;
> > >       default:
> > >               r = vhost_dev_ioctl(&v->vdev, cmd, argp);
> > >               if (r == -ENOIOCTLCMD) diff --git
> > > a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h index
> > > cab645d4a645..c7e47b29bf61 100644
> > > --- a/include/uapi/linux/vhost.h
> > > +++ b/include/uapi/linux/vhost.h
> > > @@ -171,4 +171,18 @@
> > >  #define VHOST_VDPA_SET_GROUP_ASID    _IOW(VHOST_VIRTIO, 0x7C,
> \
> > >                                            struct vhost_vring_state)
> > >
> > > +/* Stop or resume a device so it does not process virtqueue
> > > +requests anymore
> > > + *
> > > + * After the return of ioctl with stop != 0, the device must finish
> > > +any
> > > + * pending operations like in flight requests. It must also
> > > +preserve all
> > > + * the necessary state (the virtqueue vring base plus the possible
> > > +device
> > > + * specific states) that is required for restoring in the future.
> > > +The
> > > + * device must not change its configuration after that point.
> > > + *
> > > + * After the return of ioctl with stop == 0, the device can
> > > +continue
> > > + * processing buffers as long as typical conditions are met (vq is
> > > +enabled,
> > > + * DRIVER_OK status bit is enabled, etc).
> > > + */
> > > +#define VHOST_VDPA_STOP                      _IOW(VHOST_VIRTIO, 0x7D, int)
> > > +
A better name is VHOST_VDPA_SET_STATE
State = stop/suspend
State = start/resume

Suspend/resume seems more logical, as opposed start/stop, because it more clearly indicates that the resume (start) is from some programmed beginning (and not first boot).

> > >  #endif
> >
> > I wonder how does this interact with the admin vq idea.
> > I.e. if we stop all VQs then apparently admin vq can't work either ...
> > Thoughts?
> >
> 
> Copying here the answer to Parav, feel free to answer to any thread or
> highlight if I missed something :). Using the admin vq proposal terminology of
> "device group".
> 
> --
> This would stop a device of a device
> group, but not the whole virtqueue group. If the admin VQ is offered by the
> PF (since it's not exposed to the guest), it will continue accepting requests as
> normal. If it's exposed in the VF, I think the best bet is to shadow it, since
> guest and host requests could conflict.
> 

vhost-vdpa device is exposed for a VF through vp-vdpa driver to user land.
Now vp-vdpa driver will have to choose between using config register vs using AQ to suspend/resume the device.

Why not always begin with more superior interface of AQ that address multiple of these needs for LM case?

For LM case, more you explore, we realize that either VF relying on PF's AQ for query/config/setup/restore makes more sense or have its own dedicated AQ.

VM's suspend/resume operation can be handled through the shadow Q.

> Since this is offered through vdpa, the device backend driver can route it to
> whatever method works better for the hardware. For example, to send an
> admin vq command to the PF. That's why it's important to keep the feature
> as self-contained and orthogonal to others as possible.
> --
> 
> > > --
> > > 2.31.1
> >