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From:   Jason Wang <jasowang@redhat.com>
Date:   Fri, 27 May 2022 10:26:35 +0800
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Subject: Re: [PATCH v4 0/4] Implement vdpasim stop operation
To:     Parav Pandit <parav@nvidia.com>
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On Thu, May 26, 2022 at 8:54 PM Parav Pandit <parav@nvidia.com> wrote:
>
>
>
> > From: Eugenio P=C3=A9rez <eperezma@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 !=3D 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 =3D=3D 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=E2=80=
=99t 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.

>
> Why can't we use this ioctl() to indicate driver to start/stop the device=
 instead of driving it through the driver_ok?

So the idea is to add capability that does not exist in the spec. Then
came the stop/resume which can't be done via DRIVER_OK. I think we
should only allow the stop/resume to succeed after DRIVER_OK is set.

> This is in the context of other discussion we had in the LM series.

Do you see any issue that blocks the live migration?

Thanks