> From: Eugenio Perez Martin <eperezma at redhat.com>
> Sent: Friday, May 27, 2022 3:55 AM
>
> On Fri, May 27, 2022 at 4:26 AM Jason Wang <jasowang at redhat.com>
wrote:
> >
> > On Thu, May 26, 2022 at 8:54 PM Parav Pandit <parav at
nvidia.com> wrote:
> > >
> > >
> > >
> > > > From: Eugenio P?rez <eperezma at redhat.com>
> > > > Sent: Thursday, May 26, 2022 8:44 AM
> > >
> > > > 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.
> > > >
> > > >
> > > >
> > > > 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).
> > >
> > > Just to be clear, we are adding vdpa level new ioctl() that
doesn?t map to
> any mechanism in the virtio spec.
> >
> > We try to provide forward compatibility to VIRTIO_CONFIG_S_STOP. That
> > means it is expected to implement at least a subset of
> > VIRTIO_CONFIG_S_STOP.
> >
>
> Appending a link to the proposal, just for reference [1].
>
> > >
> > > Why can't we use this ioctl() to indicate driver to
start/stop the device
> instead of driving it through the driver_ok?
> >
>
> Parav, I'm not sure I follow you here.
>
> By the proposal, the resume of the device is (From qemu POV):
> 1. To configure all data vqs and cvq (addr, num, ...) 2. To enable only
CVQ, not
> data vqs 3. To send DRIVER_OK 4. Wait for all buffers of CVQ to be used 5.
To
> enable all others data vqs (individual ioctl at the moment)
>
> Where can we fit the resume (as "stop(false)") here? If the
device is stopped
> (as if we send stop(true) before DRIVER_OK), we don't read CVQ first.
If we
> send it right after (or instead) DRIVER_OK, data buffers can reach data vqs
> before configuring RSS.
>
It doesn?t make sense with currently proposed way of using cvq to replay the
config.
Need to continue with currently proposed temporary method that subsequently to
be replaced with optimized flow as we discussed.