> From: Chengwen Feng [mailto:fengcheng...@huawei.com]
> Sent: Thursday, 11 April 2024 14.04
>
> 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: sta...@dpdk.org
>
> Signed-off-by: Chengwen Feng <fengcheng...@huawei.com>
> Signed-off-by: Dengdui Huang <huangdeng...@huawei.com>
>
> ---
> v3: fix checkpatch warning "missing --in-reply-to".
> v2: add RTE_ATOMIC(uint64_t) wrap which address Morten's comment.
>
> ---
> 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,
You are right; old_link has local scope and is on the stack, so atomic store is
not required.
And since rte_eth_linkstatus_set() is an internal function called from the
driver only, it is probably safe to assume that *new_link is on the caller's
stack and doesn't change while being accessed by this function.
I guess that new_link is passed by reference for performance and
future-proofing reasons; it could have been passed by value instead. If it was
passed by value, atomic access would certainly not be required.
In other words: You are right here too; new_link does not require atomic load.
> + new_link->val64,
> + 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,
> + rte_memory_order_seq_cst);
It is not safe to assume that the link pointer points to local memory on the
caller's stack.
The link pointer might point to a shared memory area, used by multiple
threads/processes, so it needs to be stored atomically using
rte_atomic_store_explicit(&link->val64, ..., rte_memory_order_seq_cst).
> }
>
> /**
> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> index 147257d6a2..ccf43e468a 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 {
> + RTE_ATOMIC(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
