[PATCH] ethdev: fix strict aliasing lead to link cannot be up

[Morten Brørup]

> From: Chengwen Feng [mailto:fengchengwen at huawei.com]
> Sent: Thursday, 11 April 2024 05.08
> 
> Fix a problem introduced by a compiler upgrade (from gcc10 to gcc12.3),
> which will lead the hns3 NIC can't link up. The root cause is strict
> aliasing violation in rte_eth_linkstatus_set() with hns3 driver, see
> [1] for more details.
> 
> This commit use union to avoid such aliasing violation.
> 
> [1] Strict aliasing problem with rte_eth_linkstatus_set()
>     https://marc.info/?l=dpdk-dev&m=171274148514777&w=3
> 
> Cc: stable at dpdk.org
> 
> Signed-off-by: Chengwen Feng <fengchengwen at huawei.com>
> Signed-off-by: Dengdui Huang <huangdengdui at huawei.com>
> ---

The patch mixes atomic and non-atomic access.
This is not new for DPDK, which used to rely on compiler built-in atomics.

I'm not sure if it needs to be changed, but my suggestion is inline below.

>  lib/ethdev/ethdev_driver.h | 23 +++++++----------------
>  lib/ethdev/rte_ethdev.h    | 16 ++++++++++------
>  2 files changed, 17 insertions(+), 22 deletions(-)
> 
> diff --git a/lib/ethdev/ethdev_driver.h b/lib/ethdev/ethdev_driver.h
> index 0dbf2dd6a2..9d831d5c84 100644
> --- a/lib/ethdev/ethdev_driver.h
> +++ b/lib/ethdev/ethdev_driver.h
> @@ -1674,18 +1674,13 @@ static inline int
>  rte_eth_linkstatus_set(struct rte_eth_dev *dev,
>  		       const struct rte_eth_link *new_link)
>  {
> -	RTE_ATOMIC(uint64_t) *dev_link = (uint64_t __rte_atomic *)&(dev-
> >data->dev_link);
> -	union {
> -		uint64_t val64;
> -		struct rte_eth_link link;
> -	} orig;
> -
> -	RTE_BUILD_BUG_ON(sizeof(*new_link) != sizeof(uint64_t));
> +	struct rte_eth_link old_link;
> 
> -	orig.val64 = rte_atomic_exchange_explicit(dev_link, *(const
> uint64_t *)new_link,
> -					rte_memory_order_seq_cst);
> +	old_link.val64 = rte_atomic_exchange_explicit(&dev->data-
> >dev_link.val64,

old_link.val64 should be written using:
rte_atomic_store_explicit(&old_link.val64, rte_atomic_exchange_explicit(dev_link, ...), rte_memory_order_seq_cst)

> +						      new_link->val64,

new_link->val64 should be read using:
rte_atomic_load_explicit(&new_link->val64, rte_memory_order_seq_cst)

> +						      rte_memory_order_seq_cst);

> 
> -	return (orig.link.link_status == new_link->link_status) ? -1 : 0;
> +	return (old_link.link_status == new_link->link_status) ? -1 : 0;
>  }
> 
>  /**
> @@ -1701,12 +1696,8 @@ static inline void
>  rte_eth_linkstatus_get(const struct rte_eth_dev *dev,
>  		       struct rte_eth_link *link)
>  {
> -	RTE_ATOMIC(uint64_t) *src = (uint64_t __rte_atomic *)&(dev->data-
> >dev_link);
> -	uint64_t *dst = (uint64_t *)link;
> -
> -	RTE_BUILD_BUG_ON(sizeof(*link) != sizeof(uint64_t));
> -
> -	*dst = rte_atomic_load_explicit(src, rte_memory_order_seq_cst);
> +	link->val64 = rte_atomic_load_explicit(&dev->data->dev_link.val64,

link->val64 should be written using:
rte_atomic_store_explicit(&link->val64, rte_atomic_load_explicit(&dev->data->dev_link.val64, ...), rte_memory_order_seq_cst)


> +					       rte_memory_order_seq_cst);
>  }
> 
>  /**
> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> index 147257d6a2..0b5d3d2318 100644
> --- a/lib/ethdev/rte_ethdev.h
> +++ b/lib/ethdev/rte_ethdev.h
> @@ -332,12 +332,16 @@ struct rte_eth_stats {
>  /**
>   * A structure used to retrieve link-level information of an Ethernet
> port.
>   */
> -__extension__
> -struct __rte_aligned(8) rte_eth_link { /**< aligned for atomic64
> read/write */
> -	uint32_t link_speed;        /**< RTE_ETH_SPEED_NUM_ */
> -	uint16_t link_duplex  : 1;  /**< RTE_ETH_LINK_[HALF/FULL]_DUPLEX
> */
> -	uint16_t link_autoneg : 1;  /**< RTE_ETH_LINK_[AUTONEG/FIXED] */
> -	uint16_t link_status  : 1;  /**< RTE_ETH_LINK_[DOWN/UP] */
> +struct rte_eth_link {
> +	union {
> +		uint64_t val64; /**< used for atomic64 read/write */
> +		struct {
> +			uint32_t link_speed;	    /**< RTE_ETH_SPEED_NUM_
> */
> +			uint16_t link_duplex  : 1;  /**<
> RTE_ETH_LINK_[HALF/FULL]_DUPLEX */
> +			uint16_t link_autoneg : 1;  /**<
> RTE_ETH_LINK_[AUTONEG/FIXED] */
> +			uint16_t link_status  : 1;  /**<
> RTE_ETH_LINK_[DOWN/UP] */
> +		};
> +	};
>  };
> 
>  /**@{@name Link negotiation
> --
> 2.17.1