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

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

On Fri, Sep 14, 2018 at 01:10:07PM +0100, Ferruh Yigit wrote:
> On 9/10/2018 8:36 AM, Tiwei Bie wrote:
> > 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
> > 
> 
> Getting this error with meson build system, since "-fno-strict-aliasing" is not
> added to meson build file.

Thanks! I didn't notice this. I just tried the meson build manually.
The warning isn't treated as errors on my side. So it was not caught
by the build script..

> BUT instead of disabling compiler warning,
> 
> first element of the "struct vring_used_elem ring[0]" is "uint32_t id;", so it
> should be accessed as uint32_t. What about following:
> 
>   #define vhost_avail_event(vr) \
>  -       (*(volatile uint16_t*)&(vr)->used->ring[(vr)->size])
>  +       (*(volatile uint32_t*)&(vr)->used->ring[(vr)->size])
> 
> And for "vhost_avail_event(vq) = vq->last_avail_idx;", "last_avail_idx" is
> uint16_t, so assigning from uint16_t to uint32_t should be ok too.

At the end of used ring, there is a 16bits event_idx. It can't
be interpreted as a vring_used_elem and it's also not part of a
vring_used_elem. This is the traditional way how we access it:

https://github.com/torvalds/linux/blob/a06b0c82a049/drivers/vhost/vhost.c#L50
https://github.com/freebsd/freebsd/blob/209b94df8701/sys/dev/virtio/virtio_ring.h#L133

> 
> If above works for you please send a patch for update also removing
> "-fno-strict-aliasing", this can be rebased on existing patch.
> 
> If not works, please at least send a patch to add flag to meson build file

I'll send a patch to add flag to meson build file.

Thanks!

> 
> Thanks,
> ferruh
> 
> >>
> >>>   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])
> >>>
>