Re: [dpdk-dev] [PATCH] vhost: fix vhost user virtqueue not accessable

[Adrian Moreno <amorenoz@redhat.com>]

Hi Marvin,

On 10/25/19 6:20 PM, Marvin Liu wrote:
> Log feature is disabled in vhost user, so that log address was invalid
> when checking. Add feature bit check can skip useless address check.
>
Just so I understand, what conditions is the log address invalid?

> Fixes: 04cfc7fdbfca ("vhost: translate incoming log address to gpa")
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> ---
>  lib/librte_vhost/vhost_user.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index 61ef699ac..0407fdc29 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -741,13 +741,15 @@ translate_ring_addresses(struct virtio_net *dev, int vq_index)
>  		vq->last_avail_idx = vq->used->idx;
>  	}
>  
> -	vq->log_guest_addr =
> -		translate_log_addr(dev, vq, addr->log_guest_addr);
> -	if (vq->log_guest_addr == 0) {
> -		RTE_LOG(DEBUG, VHOST_CONFIG,
> -			"(%d) failed to map log_guest_addr .\n",
> -			dev->vid);
> -		return dev;
> +	if (dev->features & (1ULL << VHOST_F_LOG_ALL)) {
> +		vq->log_guest_addr =
> +			translate_log_addr(dev, vq, addr->log_guest_addr);

VHOST_F_LOG_ALL is only negotiated once the migration has started (at least from
qemu's perspective).
That means that we will postponing the translation  of the log address to the
vhost_user_set_vring_addr() call that follows the VHOST_F_LOG_ALL enabling. In
that call there are (at least) two things that could go wrong and lead to a
migration failure:
- If VHOST_USER_F_PROTOCOL_FEATURES is not enabled, the address won't be translated:

vhost_user:795
	if ((vq->enabled && (dev->features &
				(1ULL << VHOST_USER_F_PROTOCOL_FEATURES))) ||
			access_ok) {
		dev = translate_ring_addresses(dev, msg->payload.addr.index);
		if (!dev)
			return RTE_VHOST_MSG_RESULT_ERR;

		*pdev = dev;
	}

- If the IOMMU is enabled and there's a miss, we would have to wait for the
IOTLB_UPDATE and during that time, there would be failed accesses to the (still
untranslated) log address.



> +		if (vq->log_guest_addr == 0) {
> +			RTE_LOG(DEBUG, VHOST_CONFIG,
> +				"(%d) failed to map log_guest_addr .\n",
> +				dev->vid);
> +			return dev;
> +		}
>  	}
>  	vq->access_ok = 1;
>  
> 
Thanks,
Adrian