Re: [PATCH v2 2/6] powerpc64/bpf: Support tailcalls with subprogs
From: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Date: 2026-01-14 12:27:52
Also in:
bpf, linux-kselftest, lkml
Le 14/01/2026 à 12:44, adubey@linux.ibm.com a écrit :
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From: Abhishek Dubey <redacted> 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 <redacted> --- 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() ?
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/* * 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 *
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/* 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.
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+int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx); #endif #endifdiff --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));