[PATCH v2 2/6] powerpc64/bpf: Support tailcalls with subprogs

adubey
2026-01-14T11:46:19Z

From: Abhishek Dubey <adubey@linux.ibm.com>

Enabling tailcalls with subprog combinations by referencing
method. The actual tailcall count is always maintained in the
tail_call_info variable present in the frame of main function
(also called entry function). The tail_call_info variables in
the frames of all other subprog contains reference to the
tail_call_info present in frame of main function.

Dynamic resolution interprets the tail_call_info either as
value or reference depending on the context of active frame
while tailcall is invoked.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
---
arch/powerpc/net/bpf_jit.h | 12 +++++-
arch/powerpc/net/bpf_jit_comp.c | 10 ++++-
arch/powerpc/net/bpf_jit_comp64.c | 68 +++++++++++++++++++++++--------
3 files changed, 70 insertions(+), 20 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
index 45d419c0ee73..5d735bc5e6bd 100644
--- a/arch/powerpc/net/bpf_jit.h
+++ b/arch/powerpc/net/bpf_jit.h
@@ -51,6 +51,12 @@
EMIT(PPC_INST_BRANCH_COND | (((cond) & 0x3ff) << 16) | (offset & 0xfffc)); \
} while (0)

+/* Same as PPC_BCC_SHORT, except valid dest is known prior to call. */
+#define PPC_COND_BRANCH(cond, dest) \
+ do { \
+ long offset = (long)(dest) - CTX_NIA(ctx); \
+ EMIT(PPC_INST_BRANCH_COND | (((cond) & 0x3ff) << 16) | (offset & 0xfffc)); \
+ } while (0)
/*
* Sign-extended 32-bit immediate load
*
@@ -75,6 +81,8 @@

/* for tailcall counter */
#define BPF_PPC_TAILCALL 8
+/* for gpr non volatile registers BPG_REG_6 to 10 */
+#define BPF_PPC_STACK_SAVE (6*8)

/* If dummy pass (!image), account for maximum possible instructions */
#define PPC_LI64(d, i) do { \
@@ -170,6 +178,7 @@ struct codegen_context {
unsigned int alt_exit_addr;
u64 arena_vm_start;
u64 user_vm_start;
+ bool is_subprog;
};

#define bpf_to_ppc(r) (ctx->b2p[r])
@@ -204,11 +213,10 @@ void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
void bpf_jit_build_fentry_stubs(u32 *image, struct codegen_context *ctx);
void bpf_jit_realloc_regs(struct codegen_context *ctx);
int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx, int tmp_reg, long exit_addr);
-
int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, u32 *fimage, int pass,
struct codegen_context *ctx, int insn_idx,
int jmp_off, int dst_reg, u32 code);
-
+int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx);
#endif

#endif
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 5e976730b2f5..069a8822c30d 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -206,6 +206,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
cgctx.stack_size = round_up(fp->aux->stack_depth, 16);
cgctx.arena_vm_start = bpf_arena_get_kern_vm_start(fp->aux->arena);
cgctx.user_vm_start = bpf_arena_get_user_vm_start(fp->aux->arena);
+ cgctx.is_subprog = bpf_is_subprog(fp);

/* Scouting faux-generate pass 0 */
if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
@@ -435,6 +436,11 @@ void bpf_jit_free(struct bpf_prog *fp)
bpf_prog_unlock_free(fp);
}

+bool bpf_jit_supports_subprog_tailcalls(void)
+{
+ return IS_ENABLED(CONFIG_PPC64);
+}
+
bool bpf_jit_supports_kfunc_call(void)
{
return true;
@@ -604,7 +610,7 @@ static void bpf_trampoline_setup_tail_call_cnt(u32 *image, struct codegen_contex
int func_frame_offset, int r4_off)
{
if (IS_ENABLED(CONFIG_PPC64)) {
- /* See bpf_jit_stack_tailcallcnt() */
+ /* See bpf_jit_stack_tailcallinfo_offset() */
int tailcallcnt_offset = 7 * 8;

EMIT(PPC_RAW_LL(_R3, _R1, func_frame_offset - tailcallcnt_offset));
@@ -619,7 +625,7 @@ static void bpf_trampoline_restore_tail_call_cnt(u32 *image, struct codegen_cont
int func_frame_offset, int r4_off)
{
if (IS_ENABLED(CONFIG_PPC64)) {
- /* See bpf_jit_stack_tailcallcnt() */
+ /* See bpf_jit_stack_tailcallinfo_offset() */
int tailcallcnt_offset = 7 * 8;

EMIT(PPC_RAW_LL(_R3, _R1, -tailcallcnt_offset));
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 39061cd742c1..cebf81fbd59f 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -26,8 +26,12 @@
* Ensure the top half (upto local_tmp_var) stays consistent
* with our redzone usage.
*
+ * tail_call_info - stores tailcall count value in main program's
+ * frame, stores reference to tail_call_info of
+ * main's frame in sub-prog's frame.
+ *
* [ prev sp ] <-------------
- * [ tail_call_cnt ] 8 |
+ * [ tail_call_info ] 8 |
* [ nv gpr save area ] 6*8 |
* [ local_tmp_var ] 24 |
* fp (r31) --> [ ebpf stack space ] upto 512 |
@@ -35,8 +39,6 @@
* sp (r1) ---> [ stack pointer ] --------------
*/

-/* for gpr non volatile registers BPG_REG_6 to 10 */
-#define BPF_PPC_STACK_SAVE (6*8)
/* for bpf JIT code internal usage */
#define BPF_PPC_STACK_LOCALS 24
/* stack frame excluding BPF stack, ensure this is quadword aligned */
@@ -98,7 +100,7 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
* [ prev sp ] <-------------
* [ ... ] |
* sp (r1) ---> [ stack pointer ] --------------
- * [ tail_call_cnt ] 8
+ * [ tail_call_info ] 8
* [ nv gpr save area ] 6*8
* [ local_tmp_var ] 24
* [ unused red zone ] 224
@@ -114,7 +116,7 @@ static int bpf_jit_stack_local(struct codegen_context *ctx)
}
}

-static int bpf_jit_stack_tailcallcnt(struct codegen_context *ctx)
+int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx)
{
return bpf_jit_stack_local(ctx) + BPF_PPC_STACK_LOCALS + BPF_PPC_STACK_SAVE;
}
@@ -147,17 +149,32 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
#endif

/*
- * Initialize tail_call_cnt if we do tail calls.
- * Otherwise, put in NOPs so that it can be skipped when we are
- * invoked through a tail call.
+ * Tail call count(tcc) is saved & updated only in main
+ * program's frame and the address of tcc in main program's
+ * frame (tcc_ptr) is saved in subprogs frame.
+ *
+ * Offset of tail_call_info on any frame will be interpreted
+ * as either tcc_ptr or tcc value depending on whether it is
+ * greater than MAX_TAIL_CALL_CNT or not.
*/
- if (ctx->seen & SEEN_TAILCALL) {
+ if (!ctx->is_subprog) {
EMIT(PPC_RAW_LI(bpf_to_ppc(TMP_REG_1), 0));
/* this goes in the redzone */
EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), _R1, -(BPF_PPC_TAILCALL)));
} else {
- EMIT(PPC_RAW_NOP());
- EMIT(PPC_RAW_NOP());
+ /*
+ * if tail_call_info < MAX_TAIL_CALL_CNT
+ * main prog calling first subprog -> copy reference
+ * else
+ * subsequent subprog calling another subprog -> directly copy content
+ */
+ EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_2), _R1, 0));
+ EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), bpf_to_ppc(TMP_REG_2), -(BPF_PPC_TAILCALL)));
+ EMIT(PPC_RAW_CMPLWI(bpf_to_ppc(TMP_REG_1), MAX_TAIL_CALL_CNT));
+ PPC_COND_BRANCH(COND_GT, CTX_NIA(ctx) + 8);
+ EMIT(PPC_RAW_ADDI(bpf_to_ppc(TMP_REG_1), bpf_to_ppc(TMP_REG_2),
+ -(BPF_PPC_TAILCALL)));
+ EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), _R1, -(BPF_PPC_TAILCALL)));
}

if (bpf_has_stack_frame(ctx)) {
@@ -352,19 +369,38 @@ static int bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 o
EMIT(PPC_RAW_CMPLW(b2p_index, bpf_to_ppc(TMP_REG_1)));
PPC_BCC_SHORT(COND_GE, out);

+ EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R1, bpf_jit_stack_tailcallinfo_offset(ctx)));
+ EMIT(PPC_RAW_CMPLWI(bpf_to_ppc(TMP_REG_1), MAX_TAIL_CALL_CNT));
+ PPC_COND_BRANCH(COND_LE, CTX_NIA(ctx) + 8);
+
+ /* dereference TMP_REG_1 */
+ EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), bpf_to_ppc(TMP_REG_1), 0));
+
/*
- * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
+ * if (tail_call_info == MAX_TAIL_CALL_CNT)
* goto out;
*/
- EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R1, bpf_jit_stack_tailcallcnt(ctx)));
EMIT(PPC_RAW_CMPLWI(bpf_to_ppc(TMP_REG_1), MAX_TAIL_CALL_CNT));
- PPC_BCC_SHORT(COND_GE, out);
+ PPC_COND_BRANCH(COND_EQ, out);

/*
- * tail_call_cnt++;
+ * tail_call_info++; <- Actual value of tcc here
*/
EMIT(PPC_RAW_ADDI(bpf_to_ppc(TMP_REG_1), bpf_to_ppc(TMP_REG_1), 1));
- EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), _R1, bpf_jit_stack_tailcallcnt(ctx)));
+
+ /*
+ * Before writing updated tail_call_info, distinguish if current frame
+ * is storing a reference to tail_call_info or actual tcc value in
+ * tail_call_info.
+ */
+ EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_2), _R1, bpf_jit_stack_tailcallinfo_offset(ctx)));
+ EMIT(PPC_RAW_CMPLWI(bpf_to_ppc(TMP_REG_2), MAX_TAIL_CALL_CNT));
+ PPC_COND_BRANCH(COND_GT, CTX_NIA(ctx) + 8);
+
+ /* First get address of tail_call_info */
+ EMIT(PPC_RAW_ADDI(bpf_to_ppc(TMP_REG_2), _R1, bpf_jit_stack_tailcallinfo_offset(ctx)));
+ /* Writeback updated value to tail_call_info */
+ EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), bpf_to_ppc(TMP_REG_2), 0));

/* prog = array->ptrs[index]; */
EMIT(PPC_RAW_MULI(bpf_to_ppc(TMP_REG_1), b2p_index, 8));
--
2.48.1