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From:   Jason Wang <jasowang@redhat.com>
Date:   Tue, 31 May 2022 14:44:25 +0800
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Subject: Re: [PATCH v4 0/4] Implement vdpasim stop operation
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Parav Pandit <parav@nvidia.com>,
        =?UTF-8?Q?Eugenio_P=C3=A9rez?= <eperezma@redhat.com>,
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On Tue, May 31, 2022 at 1:40 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, May 30, 2022 at 11:39:21AM +0800, Jason Wang wrote:
> > On Fri, May 27, 2022 at 6:56 PM Michael S. Tsirkin <mst@redhat.com> wro=
te:
> > >
> > > On Thu, May 26, 2022 at 12:54:32PM +0000, Parav Pandit 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 devic=
e.
> > > > >
> > > > >
> > > > >
> > > > > This is a must before get virtqueue indexes (base) for live migra=
tion,
> > > > >
> > > > > since the device could modify them after userland gets them. Ther=
e 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 finis=
h any
> > > > >
> > > > > pending operations like in flight requests. It must also preserve=
 all
> > > > >
> > > > > the necessary state (the virtqueue vring base plus the possible d=
evice
> > > > >
> > > > > specific states) that is required for restoring in the future. Th=
e
> > > > >
> > > > > device must not change its configuration after that point.
> > > > >
> > > > >
> > > > >
> > > > > After the return of ioctl with stop =3D=3D 0, the device can cont=
inue
> > > > >
> > > > > processing buffers as long as typical conditions are met (vq is e=
nabled,
> > > > >
> > > > > 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.
> > > >
> > > > Why can't we use this ioctl() to indicate driver to start/stop the =
device instead of driving it through the driver_ok?
> > > > This is in the context of other discussion we had in the LM series.
> > >
> > > If there's something in the spec that does this then let's use that.
> >
> > Actually, we try to propose a independent feature here:
> >
> > https://lists.oasis-open.org/archives/virtio-dev/202111/msg00020.html
> >
> > Does it make sense to you?
> >
> > Thanks
>
> But I thought the LM patches are trying to replace all that?

I'm not sure, and actually I think they are orthogonal. We need a new
state and the command to set the state could be transport specific or
a virtqueue.

As far as I know, most of the vendors have implemented this semantic.

Thanks

>
>
> > > Unfortunately the LM series seems to be stuck on moving
> > > bits around with the admin virtqueue ...
> > >
> > > --
> > > MST
> > >
>