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

Christophe Leroy (CS GROUP)
2026-01-14T12:27:52Z

Le 14/01/2026 à 12:44, adubey@linux.ibm.com a écrit :
> 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)

I don't like the idea of duplicating PPC_BCC_SHORT() to just kick the
verification out. Now we will have two macros doing the exact same thing
with one handling failure case and one ignoring failure case. There is a
big risk that one day or another someone will use the wrong macro.

Could you change bpf_jit_build_prologue() to return an int add use
PPC_BCC_SHORT() instead of that new PPC_COND_BRANCH() ?

> /*
> * 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)

Add spaces before and after the *

>
> /* 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);
> -

Not sure why this patch needs to remove those blank lines here and above.

> +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));