Re: [PATCH v2 11/13] vdpa: block migration if dev does not have _F_SUSPEND

[Eugenio Perez Martin]

On Wed, Feb 22, 2023 at 5:05 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2023/2/8 17:42, Eugenio Pérez 写道:
> > Next patches enable devices to be migrated even if vdpa netdev has not
> > been started with x-svq. However, not all devices are migratable, so we
> > need to block migration if we detect that.
> >
> > Block vhost-vdpa device migration if it does not offer _F_SUSPEND and it
> > has not been started with x-svq.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-vdpa.c | 21 +++++++++++++++++++++
> >   1 file changed, 21 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index 84a6b9690b..9d30cf9b3c 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -442,6 +442,27 @@ static int vhost_vdpa_init(struct vhost_dev *dev, void 
> > *opaque, Error **errp)
> >           return 0;
> >       }
> >
> > +    /*
> > +     * If dev->shadow_vqs_enabled at initialization that means the device 
> > has
> > +     * been started with x-svq=on, so don't block migration
> > +     */
> > +    if (dev->migration_blocker == NULL && !v->shadow_vqs_enabled) {
> > +        uint64_t backend_features;
> > +
> > +        /* We don't have dev->backend_features yet */
> > +        ret = vhost_vdpa_call(dev, VHOST_GET_BACKEND_FEATURES,
> > +                              &backend_features);
> > +        if (unlikely(ret)) {
> > +            error_setg_errno(errp, -ret, "Could not get backend features");
> > +            return ret;
> > +        }
> > +
> > +        if (!(backend_features & BIT_ULL(VHOST_BACKEND_F_SUSPEND))) {
> > +            error_setg(&dev->migration_blocker,
> > +                "vhost-vdpa backend lacks VHOST_BACKEND_F_SUSPEND 
> > feature.");
> > +        }
>
>
> I wonder why not let the device to decide? For networking device, we can
> live without suspend probably.
>

Right, but how can we know if this is a net device in init? I don't
think a switch (vhost_vdpa_get_device_id(dev)) is elegant.

If the parent device does not need to be suspended i'd go with
exposing a suspend ioctl but do nothing in the parent device. After
that, it could even choose to return an error for GET_VRING_BASE.

If we want to implement it as a fallback in qemu, I'd go for
implementing it on top of this series. There are a few operations we
could move to a device-kind specific ops.

Would it make sense to you?

Thanks!


> Thanks
>
>
> > +    }
> > +
> >       /*
> >        * Similar to VFIO, we end up pinning all guest memory and have to
> >        * disable discarding of RAM.
>