From mboxrd@z Thu Jan  1 00:00:00 1970
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Subject: Re: [PATCH 1/3] vhost: Fix return value of
 GET_VRING_BASE message
Date: Thu, 8 Oct 2015 13:54:52 +0800
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To: Tetsuya Mukawa <mukawa@igel.co.jp>
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On Wed, Aug 19, 2015 at 06:51:07PM +0900, Tetsuya Mukawa wrote:
> When vhost-user frontend sends GET_VRING_BASE, last used index of vring
> should be returned. In DPDK vhost library, 'last_used_idx' represents it.
> But the value can be over max index value.

That's true, and I guess it's a design. If you look at the linux
kernel vhost code, you will find that there is no such wrap, either.

> To return correct value to
> vhost frontend, it's needed to be masked.

So, it's the frontend job to get the right value, by wrapping it
within vring size.

	--yliu
> 
> Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
> ---
>  lib/librte_vhost/virtio-net.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
> index b520ec5..144f301 100644
> --- a/lib/librte_vhost/virtio-net.c
> +++ b/lib/librte_vhost/virtio-net.c
> @@ -661,7 +661,8 @@ get_vring_base(struct vhost_device_ctx ctx, uint32_t index,
>  
>  	state->index = index;
>  	/* State->index refers to the queue index. The txq is 1, rxq is 0. */
> -	state->num = dev->virtqueue[state->index]->last_used_idx;
> +	state->num = dev->virtqueue[state->index]->last_used_idx
> +			& (dev->virtqueue[state->index]->size - 1);
>  
>  	return 0;
>  }
> -- 
> 2.1.4