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

[Jason Wang]

On Tue, Feb 22, 2022 at 4:06 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Tue, Feb 22, 2022 at 8:41 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > 在 2022/2/17 下午4:22, Eugenio Perez Martin 写道:
> > > On Thu, Feb 17, 2022 at 7:02 AM Jason Wang <jasowang@redhat.com> wrote:
> > >> 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.
> > >>
> > > If we simply delete the check vhost_dev_set_features will return an
> > > error, failing the start of the migration. More on this below.
> >
> >
> > Ok.
> >
> >
> > >
> > >> 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?
> > >>
> > > Yes, FEATURES_OK seems more appropriate actually so I will switch to
> > > it for the next version.
> > >
> > > But it should be functionally equivalent, since
> > > vhost.c:vhost_dev_start sets both and the setting of _F_LOG_ALL cannot
> > > be concurrent with it.
> >
> >
> > Right.
> >
> >
> > >
> > >> 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.
> > >>
> > > Yes, that's the intention of the patch.
> > >
> > > We have 4 cases here:
> > > a) We're being called from vhost_dev_start, with enable_log = false
> > > b) We're being called from vhost_dev_start, with enable_log = true
> >
> >
> > And this case makes us can't simply return without calling vhost-vdpa.
> >
>
> It calls because {FEATURES,DRIVER}_OK is still not set at that point.
>
> >
> > > c) We're being called from vhost_dev_set_log, with enable_log = false
> > > d) We're being called from vhost_dev_set_log, with enable_log = true
> > >
> > > The way to tell the difference between a/b and c/d is to check if
> > > {FEATURES,DRIVER}_OK is set. And, as you point out in previous mails,
> > > F_LOG_ALL must be filtered unconditionally since SVQ tracks dirty
> > > memory through the memory unmapping, so we clear the bit
> > > unconditionally if we detect that VHOST_SET_FEATURES will be called
> > > (cases a and b).
> > >
> > > Another possibility is to track if features have been set with a bool
> > > in vhost_vdpa or something like that. But it seems cleaner to me to
> > > only store that in the actual device.
> >
> >
> > So I suggest to make sure codes match the comment:
> >
> >          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.
> >               */
> >              return 0;
> >          }
> >
> > It would be better to check whether the caller is toggling _F_LOG_ALL in
> > this case.
> >
>
> How to detect? We can save feature flags and compare, but ignoring all
> set_features after FEATURES_OK seems simpler to me.

Something like:

(status ^ status_old == _F_LOG_ALL) ?

It helps us to return errors on wrong features set during DRIVER_OK.

Thanks

>
> Would changing the comment work? Something like "set_features after
> _S_FEATURES_OK means vhost is trying to enable or disable _F_LOG, and
> the device would report wrong dirty pages. SVQ handles it."
>
> Thanks!
>
> > Thanks
> >
> >
> > >
> > >> 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);
> >
>