bpf: Introduce global percpu data - #8642
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There are many adjacent blank lines in kernel/bpf/ that have accumulated over time. Drop them for cleanup. No functional changes intended. Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
In the next commit, percpu_array map will add map_direct_value_addr support. IOW, it will add a map_type check in the iff condition of the bpf_map_direct_read() code block, which will reduce the code block readability. Hence, factor out check_map_mem_read helper to improve the readability, and the maintainability for the percpu_array map case. Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Introduce global percpu data, inspired by the commit 6316f78 ("Merge branch 'support-global-data'"). It enables the definition of global percpu variables in BPF, similar to the include/linux/percpu-defs.h::DEFINE_PER_CPU() macro. For example, in BPF, it is able to define a global percpu variable like: int data SEC(".percpu"); With this patch, tools like retsnoop [1] and bpfsnoop [2] can simplify their BPF code for handling LBRs. The code can be updated from static struct perf_branch_entry lbrs[1][MAX_LBR_ENTRIES] SEC(".data.lbrs"); to static struct perf_branch_entry lbrs[MAX_LBR_ENTRIES] SEC(".percpu.lbrs"); This eliminates the need to retrieve the CPU ID using the bpf_get_smp_processor_id() helper. Additionally, by reusing global percpu data map, sharing information between tail callers and callees or freplace callers and callees becomes simpler compared to reusing percpu_array maps. Links: [1] https://github.com/anakryiko/retsnoop [2] https://github.com/bpfsnoop/bpfsnoop Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
libbpf needs a reliable way to distinguish kernels that can support global percpu data from those that cannot. Add a dedicated feature probe, so libbpf can make capability decisions early and fail predictably when global percpu data is unavailable. Acked-by: Andrii Nakryiko <andrii@kernel.org> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Add support for global percpu data in libbpf by adding a new ".percpu" section, similar to ".data". It enables efficient handling of percpu global variables in bpf programs. When generating loader for lightweight skeleton, update the percpu_array map used for global percpu data using BPF_F_ALL_CPUS, in order to update values across all CPUs using one value slot. Unlike global data, the mmaped data for global percpu data will be marked as read-only after populating the percpu_array map. Thereafter, users can read those initialized percpu data after loading prog. If they want to update the percpu data after loading prog, they have to update the percpu_array map using key=0 instead. Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Enhance bpftool to generate skeletons that properly handle global percpu
variables. The generated skeleton now includes a dedicated structure for
percpu data, allowing users to initialize and access percpu variables more
efficiently.
For global percpu variables, the skeleton now includes a nested
structure, e.g.:
struct test_global_percpu_data {
struct bpf_object_skeleton *skeleton;
struct bpf_object *obj;
struct {
struct bpf_map *percpu;
} maps;
// ...
struct test_global_percpu_data__percpu {
int data;
char run;
struct {
char set;
int i;
int nums[7];
} struct_data;
int nums[7];
} *percpu;
// ...
};
* The "struct test_global_percpu_data__percpu *percpu" points to
initialized data, which is actually "maps.percpu->mmaped".
* Before loading the skeleton, updating the
"struct test_global_percpu_data__percpu *percpu" modifies the initial
value of the corresponding global percpu variables.
* After loading the skeleton, "maps.percpu->mmaped" has been marked as
read-only in libbpf. If users want to update the global percpu
variables, they have to update the "maps.percpu" map instead.
* For lightweight skeleton, "lskel->percpu" will be protected by
"mprotect(p, sz, PROT_READ)".
* For subskeleton, those variables of global percpu data will be
skipped.
Acked-by: Quentin Monnet <qmo@kernel.org>
Assisted-by: Codex:gpt-5.5-xhigh
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
If the arch, like s390x, does not support percpu insn, these cases won't test global percpu data by checking FEAT_PERCPU_DATA support. The following APIs have been tested for global percpu data: 1. bpf_map__set_initial_value() 2. bpf_map__initial_value() 3. generated percpu struct pointer pointing to internal map's mmaped data 4. bpf_map__lookup_elem() for global percpu data map 5. bpf_map_lookup_elem_flags() for global percpu data map At the same time, the case is also tested with 'bpftool gen skeleton -L'. Assisted-by: Codex:gpt-5.5-xhigh Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
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Verify these two cases: 1. Direct reading the data of read-only percpu data's percpu_array map is allowed. 2. Direct writing the data of read-only percpu data's percpu_array map is disallowed. Assisted-by: Codex:gpt-5.5-xhigh Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Add two tests to verify the verifier log "R%d points to percpu_array map which cannot be used as const string\n". Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Add a test to verify that it is OK to iter the percpu_array map used for global percpu data. Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
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Pull request for series with
subject: bpf: Introduce global percpu data
version: 11
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1141641