Acked-by: Nithin Dabilpuram <ndabilpu...@marvell.com>
On Wed, Mar 27, 2024 at 2:47 PM Robin Jarry <rja...@redhat.com> wrote:
>
> In some cases, the node context data is used to store two pointers
> because the data is larger than the reserved 16 bytes. Having to define
> intermediate structures just to be able to cast is tedious. And without
> intermediate structures, casting to opaque pointers is hard without
> violating strict aliasing rules.
>
> Add an unnamed union to allow storing opaque pointers in the node
> context. Unfortunately, aligning an unnamed union that contains an array
> produces inconsistent results between C and C++. To preserve ABI/API
> compatibility in both C and C++, move all fast-path area fields into an
> unnamed struct which is cache aligned. Use __rte_cache_min_aligned to
> preserve existing alignment on architectures where cache lines are 128
> bytes.
>
> Add a static assert to ensure that the unnamed union is not larger than
> the context array (RTE_NODE_CTX_SZ).
>
> Signed-off-by: Robin Jarry <rja...@redhat.com>
> ---
>
> Notes:
> v5:
>
> * Helper functions to hide casting proved to be harder than expected.
> Naive casting may even be impossible without breaking strict aliasing
> rules. The only other option would be to use explicit memcpy calls.
> * Unnamed union tentative again. As suggested by Tyler (thank you!),
> using an intermediate unnamed struct to carry the alignment produces
> consistent ABI in C and C++.
> * Also, Tyler (thank you!) suggested that the fast path area alignment
> size may be incorrect for architectures where the cache line is not 64
> bytes. There will be a 64 bytes hole in the structure at the end of
> the unnamed struct before the zero length next nodes array. Use
> __rte_cache_min_aligned to preserve existing alignment.
>
> v4:
>
> * Replaced the unnamed union with helper inline functions.
>
> v3:
>
> * Added __extension__ to the unnamed struct inside the union.
> * Fixed C++ header checks.
> * Replaced alignas() with an explicit static_assert.
>
> lib/graph/rte_graph_worker_common.h | 27 ++++++++++++++++++++-------
> 1 file changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/lib/graph/rte_graph_worker_common.h
> b/lib/graph/rte_graph_worker_common.h
> index 36d864e2c14e..84d4997bbbf6 100644
> --- a/lib/graph/rte_graph_worker_common.h
> +++ b/lib/graph/rte_graph_worker_common.h
> @@ -12,7 +12,9 @@
> * process, enqueue and move streams of objects to the next nodes.
> */
>
> +#include <assert.h>
> #include <stdalign.h>
> +#include <stddef.h>
>
> #include <rte_common.h>
> #include <rte_cycles.h>
> @@ -111,14 +113,21 @@ struct __rte_cache_aligned rte_node {
> } dispatch;
> };
> /* Fast path area */
> + __extension__ struct __rte_cache_min_aligned {
> #define RTE_NODE_CTX_SZ 16
> - alignas(RTE_CACHE_LINE_SIZE) uint8_t ctx[RTE_NODE_CTX_SZ]; /**< Node
> Context. */
> - uint16_t size; /**< Total number of objects available. */
> - uint16_t idx; /**< Number of objects used. */
> - rte_graph_off_t off; /**< Offset of node in the graph reel. */
> - uint64_t total_cycles; /**< Cycles spent in this node. */
> - uint64_t total_calls; /**< Calls done to this node. */
> - uint64_t total_objs; /**< Objects processed by this node. */
> + union {
> + uint8_t ctx[RTE_NODE_CTX_SZ];
> + __extension__ struct {
> + void *ctx_ptr;
> + void *ctx_ptr2;
> + };
> + }; /**< Node Context. */
> + uint16_t size; /**< Total number of objects
> available. */
> + uint16_t idx; /**< Number of objects used. */
> + rte_graph_off_t off; /**< Offset of node in the graph
> reel. */
> + uint64_t total_cycles; /**< Cycles spent in this node. */
> + uint64_t total_calls; /**< Calls done to this node. */
> + uint64_t total_objs; /**< Objects processed by this node.
> */
> union {
> void **objs; /**< Array of object pointers. */
> uint64_t objs_u64;
> @@ -127,9 +136,13 @@ struct __rte_cache_aligned rte_node {
> rte_node_process_t process; /**< Process function. */
> uint64_t process_u64;
> };
> + };
> alignas(RTE_CACHE_LINE_MIN_SIZE) struct rte_node *nodes[]; /**< Next
> nodes. */
> };
>
> +static_assert(offsetof(struct rte_node, size) - offsetof(struct rte_node,
> ctx) == RTE_NODE_CTX_SZ,
> + "rte_node context must be RTE_NODE_CTX_SZ bytes exactly");
> +
> /**
> * @internal
> *
> --
> 2.44.0
>
