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

[Ferruh Yigit <ferruh.yigit@intel.com>]

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. 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.

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

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])
>>>