BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK¶
Note
BPF_MAP_TYPE_QUEUEandBPF_MAP_TYPE_STACKwere introduced in kernel version 4.20
BPF_MAP_TYPE_QUEUE provides FIFO storage and BPF_MAP_TYPE_STACK
provides LIFO storage for BPF programs. These maps support peek, pop and
push operations that are exposed to BPF programs through the respective
helpers. These operations are exposed to userspace applications using
the existing bpf syscall in the following way:
BPF_MAP_LOOKUP_ELEM-> peekBPF_MAP_LOOKUP_AND_DELETE_ELEM-> popBPF_MAP_UPDATE_ELEM-> push
BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK do not support
BPF_F_NO_PREALLOC.
Usage¶
Kernel BPF¶
bpf_map_push_elem()¶
long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags)
An element value can be added to a queue or stack using the
bpf_map_push_elem helper. The flags parameter must be set to
BPF_ANY or BPF_EXIST. If flags is set to BPF_EXIST then,
when the queue or stack is full, the oldest element will be removed to
make room for value to be added. Returns 0 on success, or
negative error in case of failure.
bpf_map_peek_elem()¶
long bpf_map_peek_elem(struct bpf_map *map, void *value)
This helper fetches an element value from a queue or stack without
removing it. Returns 0 on success, or negative error in case of
failure.
bpf_map_pop_elem()¶
long bpf_map_pop_elem(struct bpf_map *map, void *value)
This helper removes an element into value from a queue or
stack. Returns 0 on success, or negative error in case of failure.
Userspace¶
bpf_map_update_elem()¶
int bpf_map_update_elem (int fd, const void *key, const void *value, __u64 flags)
A userspace program can push value onto a queue or stack using libbpf’s
bpf_map_update_elem function. The key parameter must be set to
NULL and flags must be set to BPF_ANY or BPF_EXIST, with the
same semantics as the bpf_map_push_elem kernel helper. Returns 0 on
success, or negative error in case of failure.
bpf_map_lookup_elem()¶
int bpf_map_lookup_elem (int fd, const void *key, void *value)
A userspace program can peek at the value at the head of a queue or stack
using the libbpf bpf_map_lookup_elem function. The key parameter must be
set to NULL. Returns 0 on success, or negative error in case of
failure.
bpf_map_lookup_and_delete_elem()¶
int bpf_map_lookup_and_delete_elem (int fd, const void *key, void *value)
A userspace program can pop a value from the head of a queue or stack using
the libbpf bpf_map_lookup_and_delete_elem function. The key parameter
must be set to NULL. Returns 0 on success, or negative error in case of
failure.
Examples¶
Kernel BPF¶
This snippet shows how to declare a queue in a BPF program:
struct {
__uint(type, BPF_MAP_TYPE_QUEUE);
__type(value, __u32);
__uint(max_entries, 10);
} queue SEC(".maps");
Userspace¶
This snippet shows how to use libbpf’s low-level API to create a queue from userspace:
int create_queue()
{
return bpf_map_create(BPF_MAP_TYPE_QUEUE,
"sample_queue", /* name */
0, /* key size, must be zero */
sizeof(__u32), /* value size */
10, /* max entries */
NULL); /* create options */
}
References¶
https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/