Re: [PATCH v2 0/4] Implement vdpasim stop operation

[Eugenio Perez Martin <eperezma@redhat.com>]

On Wed, May 25, 2022 at 4:49 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Wed, May 25, 2022 at 1:06 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> >
> > Implement stop operation for vdpa_sim devices, so vhost-vdpa will offer
> > that backend feature and userspace can effectively stop the device.
> >
> > This is a must before get virtqueue indexes (base) for live migration,
> > since the device could modify them after userland gets them. There are
> > individual ways to perform that action for some devices
> > (VHOST_NET_SET_BACKEND, VHOST_VSOCK_SET_RUNNING, ...) but there was no
> > way to perform it for any vhost device (and, in particular, vhost-vdpa).
> >
> > 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.
>
> I'd suggest documenting this in the code maybe around ops->stop()?
>

I agree it'd be better to put in the source code, but both
vdpa_config_ops and ops->stop don't have a lot of space for docs.

Would it work to document at drivers/vdpa/vdpa.c:vhost_vdpa_stop() and
redirect config ops like "for more info, see vhost_vdpa_stop"?

Thanks!

> Thanks
>
> >
> > 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).
> >
> > In the future, we will provide features similar to VHOST_USER_GET_INFLIGHT_FD
> > so the device can save pending operations.
> >
> > Comments are welcome.
> >
> > v2:
> > * Replace raw _F_STOP with BIT_ULL(_F_STOP).
> > * Fix obtaining of stop ioctl arg (it was not obtained but written).
> > * Add stop to vdpa_sim_blk.
> >
> > Eugenio Pérez (4):
> >   vdpa: Add stop operation
> >   vhost-vdpa: introduce STOP backend feature bit
> >   vhost-vdpa: uAPI to stop the device
> >   vdpa_sim: Implement stop vdpa op
> >
> >  drivers/vdpa/vdpa_sim/vdpa_sim.c     | 21 +++++++++++++++++
> >  drivers/vdpa/vdpa_sim/vdpa_sim.h     |  1 +
> >  drivers/vdpa/vdpa_sim/vdpa_sim_blk.c |  3 +++
> >  drivers/vdpa/vdpa_sim/vdpa_sim_net.c |  3 +++
> >  drivers/vhost/vdpa.c                 | 34 +++++++++++++++++++++++++++-
> >  include/linux/vdpa.h                 |  6 +++++
> >  include/uapi/linux/vhost.h           |  3 +++
> >  include/uapi/linux/vhost_types.h     |  2 ++
> >  8 files changed, 72 insertions(+), 1 deletion(-)
> >
> > --
> > 2.27.0
> >
> >
>