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

[Eugenio Perez Martin]

On Wed, Feb 23, 2022 at 4:47 AM Jason Wang <jasowang@redhat.com> wrote:
>
> 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) ?
>

s/status/features/ ?

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

Do you mean to return errors in case of toggling other features than
_F_LOG_ALL, isn't it? That's interesting actually, but it seems it
forces vhost_vdpa to track acked_features too.

Actually, it seems to me vhost_dev->acked_features will retain the bad
features even on error. I'll investigate it.

Thanks!


> 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."
> >