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

[Jason Wang]

On Wed, Feb 16, 2022 at 11:54 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Tue, Feb 8, 2022 at 9:25 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > 在 2022/2/1 下午7:45, Eugenio Perez Martin 写道:
> > > 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.
> >
> >
> > Yes, but for SVQ, we can't disable dirty page tracking, isn't it? The
> > only thing is to ignore or filter out the F_LOG_ALL and pretend to be
> > enabled and disabled.
> >
>
> Yes, that's what this patch does.
>
> >
> > >
> > > 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?
> >
> >
> > I'm fine with filtering since it's much more simpler, but I fail to
> > understand why we need to check DRIVER_OK.
> >
>
> Ok maybe I can make that part more clear,
>
> Since both operations use vhost_vdpa_set_features we must just filter
> the one that actually sets or removes VHOST_F_LOG_ALL, without
> affecting other features.
>
> In practice, that means to not forward the set features after
> DRIVER_OK. The device is not expecting them anymore.

I wonder what happens if we don't do this.

So kernel had this check:

        /*
         * It's not allowed to change the features after they have
         * been negotiated.
         */
if (ops->get_status(vdpa) & VIRTIO_CONFIG_S_FEATURES_OK)
        return -EBUSY;

So is it FEATURES_OK actually?

For this patch, I wonder if the thing we need to do is to see whether
it is a enable/disable F_LOG_ALL and simply return.

Thanks

>
> Does that make more sense?
>
> Thanks!
>
> > Thanks
> >
> >
> > >
> > > 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);
> >
>