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Wed, 14 Jan 2026 12:27:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768393672; bh=DYAr8uZefUILpwfnCmaoF2xmqlYDZ09mukiO+4g9r94=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Eb5iOM4Tp80KUlMCviLbJWAoOoNYnyiQFT1orrpSLV99MIRIeR2YO0ifWLBxyBm7u bUaBvk/mRg1TkB9fZfnjEI8URwWOVhkxRatJvET3DbeWFy+P0505ZZ942nlKwdLLeM l8J9M/z1sSbdcBGN/deqmee/jpETe2QC0ZvARzNqiq5gWL1GrdZBUVaIL+IE0K+Jlb suhMbTGGyLpmKOkjAHGEtO8yRXNMlwY4IIrXmpKDKZ2N2+5/yT0afrpa4gFOidZjvb WET/8WK81T7qYwYVCq20ynmQRk5BIje35AmfzQL/M8IdZDQ/JT6Zo4x0FcIKRw4e5h 4oeocK+cyoOpg== Message-ID: <42d41a0d-9d26-4eeb-af46-200083261c09@kernel.org> Date: Wed, 14 Jan 2026 13:27:45 +0100 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/6] powerpc64/bpf: Support tailcalls with subprogs To: adubey@linux.ibm.com, bpf@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Cc: hbathini@linux.ibm.com, sachinpb@linux.ibm.com, venkat88@linux.ibm.com, andrii@kernel.org, eddyz87@gmail.com, mykolal@fb.com, ast@kernel.org, daniel@iogearbox.net, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, naveen@kernel.org, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, memxor@gmail.com, iii@linux.ibm.com, shuah@kernel.org References: <20260114114450.30405-1-adubey@linux.ibm.com> <20260114114450.30405-3-adubey@linux.ibm.com> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: <20260114114450.30405-3-adubey@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 14/01/2026 à 12:44, adubey@linux.ibm.com a écrit : > From: Abhishek Dubey > > 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 > --- > 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));