Re: [PATCH 28/31] vdpa: Expose VHOST_F_LOG_ALL on SVQ

[Eugenio Perez Martin]

On Sun, Jan 30, 2022 at 7:50 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/1/22 上午4:27, Eugenio Pérez 写道:
> > SVQ is able to log the dirty bits by itself, so let's use it to not
> > block migration.
> >
> > Also, ignore set and clear of VHOST_F_LOG_ALL on set_features if SVQ is
> > enabled. Even if the device supports it, the reports would be nonsense
> > because SVQ memory is in the qemu region.
> >
> > The log region is still allocated. Future changes might skip that, but
> > this series is already long enough.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-vdpa.c | 20 ++++++++++++++++++++
> >   1 file changed, 20 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index fb0a338baa..75090d65e8 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -1022,6 +1022,9 @@ static int vhost_vdpa_get_features(struct vhost_dev 
> > *dev, uint64_t *features)
> >       if (ret == 0 && v->shadow_vqs_enabled) {
> >           /* Filter only features that SVQ can offer to guest */
> >           vhost_svq_valid_guest_features(features);
> > +
> > +        /* Add SVQ logging capabilities */
> > +        *features |= BIT_ULL(VHOST_F_LOG_ALL);
> >       }
> >
> >       return ret;
> > @@ -1039,8 +1042,25 @@ static int vhost_vdpa_set_features(struct vhost_dev 
> > *dev,
> >
> >       if (v->shadow_vqs_enabled) {
> >           uint64_t dev_features, svq_features, acked_features;
> > +        uint8_t status = 0;
> >           bool ok;
> >
> > +        ret = vhost_vdpa_call(dev, VHOST_VDPA_GET_STATUS, &status);
> > +        if (unlikely(ret)) {
> > +            return ret;
> > +        }
> > +
> > +        if (status & VIRTIO_CONFIG_S_DRIVER_OK) {
> > +            /*
> > +             * vhost is trying to enable or disable _F_LOG, and the device
> > +             * would report wrong dirty pages. SVQ handles it.
> > +             */
>
>
> I fail to understand this comment, I'd think there's no way to disable
> dirty page tracking for SVQ.
>

vhost_log_global_{start,stop} are called at the beginning and end of
migration. To inform the device that it should start logging, they set
or clean VHOST_F_LOG_ALL at vhost_dev_set_log.

While SVQ does not use VHOST_F_LOG_ALL, it exports the feature bit so
vhost does not block migration. Maybe we need to look for another way
to do this?

Thanks!

> Thanks
>
>
> > +            return 0;
> > +        }
> > +
> > +        /* We must not ack _F_LOG if SVQ is enabled */
> > +        features &= ~BIT_ULL(VHOST_F_LOG_ALL);
> > +
> >           ret = vhost_vdpa_get_dev_features(dev, &dev_features);
> >           if (ret != 0) {
> >               error_report("Can't get vdpa device features, got (%d)", ret);
>