Re: [dpdk-dev] [PATCH] vhost: fix vhost interrupt support

[Tiwei Bie <tiwei.bie@intel.com>]

Hi Maxime,

On Mon, Sep 10, 2018 at 09:22:00AM +0200, Maxime Coquelin wrote:
> Hi Tiwei,
> 
> On 09/05/2018 01:55 AM, Tiwei Bie wrote:
> > When VIRTIO_RING_F_EVENT_IDX is negotiated, we need to
> > update the avail event to enable the notification.
> > 
> > Fixes: 3f8ff12821e4 ("vhost: support interrupt mode")
> > Cc: stable@dpdk.org
> > 
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > ---
> >   lib/librte_vhost/Makefile |  1 +
> >   lib/librte_vhost/vhost.c  | 18 ++++++++++++------
> >   lib/librte_vhost/vhost.h  |  2 ++
> >   3 files changed, 15 insertions(+), 6 deletions(-)
> > 
> > diff --git a/lib/librte_vhost/Makefile b/lib/librte_vhost/Makefile
> > index de431fbb7..531cf4832 100644
> > --- a/lib/librte_vhost/Makefile
> > +++ b/lib/librte_vhost/Makefile
> > @@ -13,6 +13,7 @@ LIBABIVER := 4
> >   CFLAGS += -DALLOW_EXPERIMENTAL_API
> >   CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) -O3 -D_FILE_OFFSET_BITS=64
> >   CFLAGS += -I vhost_user
> > +CFLAGS += -fno-strict-aliasing
> 
> I'm not clear why this is needed looking at the code below,
> could you please explain?

Without this, we will get below build error:

lib/librte_vhost/vhost.c: In function ‘vhost_enable_notify_split’:
lib/librte_vhost/vhost.h:656:4: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  (*(volatile uint16_t*)&(vr)->used->ring[(vr)->size])
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/librte_vhost/vhost.c:659:4: note: in expansion of macro ‘vhost_avail_event’
    vhost_avail_event(vq) = vq->last_avail_idx;
    ^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

> 
> >   LDLIBS += -lpthread
> >   ifeq ($(CONFIG_RTE_LIBRTE_VHOST_NUMA),y)
> > diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
> > index 3c9be10a0..88b1781d5 100644
> > --- a/lib/librte_vhost/vhost.c
> > +++ b/lib/librte_vhost/vhost.c
> > @@ -646,12 +646,18 @@ rte_vhost_avail_entries(int vid, uint16_t queue_id)
> >   }
> >   static inline void
> > -vhost_enable_notify_split(struct vhost_virtqueue *vq, int enable)
> > +vhost_enable_notify_split(struct virtio_net *dev,
> > +		struct vhost_virtqueue *vq, int enable)
> >   {
> > -	if (enable)
> > -		vq->used->flags &= ~VRING_USED_F_NO_NOTIFY;
> > -	else
> > -		vq->used->flags |= VRING_USED_F_NO_NOTIFY;
> > +	if (!(dev->features & (1ULL << VIRTIO_RING_F_EVENT_IDX))) {
> > +		if (enable)
> > +			vq->used->flags &= ~VRING_USED_F_NO_NOTIFY;
> > +		else
> > +			vq->used->flags |= VRING_USED_F_NO_NOTIFY;
> > +	} else {
> > +		if (enable)
> > +			vhost_avail_event(vq) = vq->last_avail_idx;
> > +	}
> >   }
> >   static inline void
> > @@ -689,7 +695,7 @@ rte_vhost_enable_guest_notification(int vid, uint16_t queue_id, int enable)
> >   	if (vq_is_packed(dev))
> >   		vhost_enable_notify_packed(dev, vq, enable);
> >   	else
> > -		vhost_enable_notify_split(vq, enable);
> > +		vhost_enable_notify_split(dev, vq, enable);
> >   	return 0;
> >   }
> > diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> > index 760a09c0d..25ffd7614 100644
> > --- a/lib/librte_vhost/vhost.h
> > +++ b/lib/librte_vhost/vhost.h
> > @@ -648,6 +648,8 @@ vhost_iova_to_vva(struct virtio_net *dev, struct vhost_virtqueue *vq,
> >   	return __vhost_iova_to_vva(dev, vq, iova, len, perm);
> >   }
> > +#define vhost_avail_event(vr) \
> > +	(*(volatile uint16_t*)&(vr)->used->ring[(vr)->size])
> >   #define vhost_used_event(vr) \
> >   	(*(volatile uint16_t*)&(vr)->avail->ring[(vr)->size])
> >