Subject: Re: [PATCH v2] powerpc64/bpf: Add powerpc64 JIT support for timed may_goto

From: Saket Kumar Bhaskar <[email-masked]>
Date: Fri, 3 Apr 2026 15:13:15 +0530

On Fri, Apr 03, 2026 at 01:14:09AM +0530, Hari Bathini wrote:
> 
> 
> On 02/04/26 9:40 pm, Saket Kumar Bhaskar wrote:
> > When verifier sees a timed may_goto instruction, it emits a call to
> > arch_bpf_timed_may_goto() with a stack offset in BPF_REG_AX
> > (powerpc64 R12) and expects the refreshed count value to be returned
> > in the same register. The verifier doesn't save or restore any registers
> > before emitting this call.
> > 
> > arch_bpf_timed_may_goto() should act as a trampoline to call
> > bpf_check_timed_may_goto() with powerpc64 ELF ABI calling convention.
> > 
> > To support this custom calling convention, implement
> > arch_bpf_timed_may_goto() in assembly and make sure BPF caller saved
> > registers are preserved, then call bpf_check_timed_may_goto with
> > the powerpc64 ABI calling convention where first argument and return
> > value both are in R3. Finally, move the result back into BPF_REG_AX(R12)
> > before returning.
> > 
> > Also, introduce bpf_jit_emit_func_call() that computes the offset from
> > kernel_toc_addr(), validates that the target and emits the ADDIS/ADDI
> > sequence to load the function address before performing the indirect
> > branch via MTCTR/BCTRL. The existing code in bpf_jit_emit_func_call_rel()
> > is refactored to use this function.
> > 
> 
> Looks good to me.
> 
> Acked-by: Hari Bathini <[email-masked]>
> 
> > Signed-off-by: Saket Kumar Bhaskar <[email-masked]>
> > ---
> > This patch has been rebased on top of these 3 patches on powerpc-next in
> > the following sequence:
> > 1. https://lore.kernel.org/bpf/[email-masked]/
> > 2. https://lore.kernel.org/bpf/[email-masked]/
> > 3. https://lore.kernel.org/bpf/[email-masked]/
> > 
> > Changes since v1:
> > - Compile bpf_timed_may_goto.o conditionally for PPC64. (Hari)
> > - Moved reladdr at the beginning of the function. (Hari)
> > - Added error check for bpf_jit_emit_func_call() in
> >    bpf_jit_build_body(). (Hari)
> > - Save registers after 32 bytes as MIN_FRAME_SIZE is 32 bytes for
> >    powerpc ABIv2. (Hari)
> > ---
> >   arch/powerpc/net/Makefile             |  4 ++
> >   arch/powerpc/net/bpf_jit_comp.c       |  5 +++
> >   arch/powerpc/net/bpf_jit_comp64.c     | 62 ++++++++++++++++++++++-----
> >   arch/powerpc/net/bpf_timed_may_goto.S | 57 ++++++++++++++++++++++++
> >   4 files changed, 117 insertions(+), 11 deletions(-)
> >   create mode 100644 arch/powerpc/net/bpf_timed_may_goto.S
> > 
> > diff --git a/arch/powerpc/net/Makefile b/arch/powerpc/net/Makefile
> > index 8e60af32e51e..87395d7e2672 100644
> > --- a/arch/powerpc/net/Makefile
> > +++ b/arch/powerpc/net/Makefile
> > @@ -3,3 +3,7 @@
> >   # Arch-specific network modules
> >   #
> >   obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o bpf_jit_comp$(BITS).o
> > +
> > +ifdef CONFIG_PPC64
> > ++obj-$(CONFIG_BPF_JIT) += bpf_timed_may_goto.o
Apologies, a stray '+' slipped into the patch. I'll send a corrected respin shortly.

-Saket
> > +endif
> > diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> > index 50103b3794fb..9b2b456b0765 100644
> > --- a/arch/powerpc/net/bpf_jit_comp.c
> > +++ b/arch/powerpc/net/bpf_jit_comp.c
> > @@ -537,6 +537,11 @@ bool bpf_jit_supports_subprog_tailcalls(void)
> >   	return IS_ENABLED(CONFIG_PPC64);
> >   }
> > +bool bpf_jit_supports_timed_may_goto(void)
> > +{
> > +	return IS_ENABLED(CONFIG_PPC64);
> > +}
> > +
> >   bool bpf_jit_supports_kfunc_call(void)
> >   {
> >   	return IS_ENABLED(CONFIG_PPC64);
> > diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> > index db364d9083e7..dab106cae22b 100644
> > --- a/arch/powerpc/net/bpf_jit_comp64.c
> > +++ b/arch/powerpc/net/bpf_jit_comp64.c
> > @@ -451,10 +451,28 @@ void arch_bpf_stack_walk(bool (*consume_fn)(void *, u64, u64, u64), void *cookie
> >   	}
> >   }
> > +static int bpf_jit_emit_func_call(u32 *image, struct codegen_context *ctx, u64 func_addr, int reg)
> > +{
> > +	long reladdr = func_addr - kernel_toc_addr();
> > +
> > +	if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
> > +		pr_err("eBPF: address of %ps out of range of kernel_toc.\n", (void *)func_addr);
> > +		return -ERANGE;
> > +	}
> > +
> > +	EMIT(PPC_RAW_ADDIS(reg, _R2, PPC_HA(reladdr)));
> > +	EMIT(PPC_RAW_ADDI(reg, reg, PPC_LO(reladdr)));
> > +	EMIT(PPC_RAW_MTCTR(reg));
> > +	EMIT(PPC_RAW_BCTRL());
> > +
> > +	return 0;
> > +}
> > +
> >   int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *ctx, u64 func)
> >   {
> >   	unsigned long func_addr = func ? ppc_function_entry((void *)func) : 0;
> > -	long reladdr;
> > +	long __maybe_unused reladdr;
> > +	int ret;
> >   	/* bpf to bpf call, func is not known in the initial pass. Emit 5 nops as a placeholder */
> >   	if (!func) {
> > @@ -507,16 +525,9 @@ int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *
> >   	EMIT(PPC_RAW_BCTRL());
> >   #else
> >   	if (core_kernel_text(func_addr)) {
> > -		reladdr = func_addr - kernel_toc_addr();
> > -		if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
> > -			pr_err("eBPF: address of %ps out of range of kernel_toc.\n", (void *)func);
> > -			return -ERANGE;
> > -		}
> > -
> > -		EMIT(PPC_RAW_ADDIS(_R12, _R2, PPC_HA(reladdr)));
> > -		EMIT(PPC_RAW_ADDI(_R12, _R12, PPC_LO(reladdr)));
> > -		EMIT(PPC_RAW_MTCTR(_R12));
> > -		EMIT(PPC_RAW_BCTRL());
> > +		ret = bpf_jit_emit_func_call(image, ctx, func_addr, _R12);
> > +		if (ret)
> > +			return ret;
> >   	} else {
> >   		if (IS_ENABLED(CONFIG_PPC64_ELF_ABI_V1)) {
> >   			/* func points to the function descriptor */
> > @@ -1755,6 +1766,35 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
> >   			if (ret < 0)
> >   				return ret;
> > +			/*
> > +			 * Call to arch_bpf_timed_may_goto() is emitted by the
> > +			 * verifier and called with custom calling convention with
> > +			 * first argument and return value in BPF_REG_AX (_R12).
> > +			 *
> > +			 * The generic helper or bpf function call emission path
> > +			 * may use the same scratch register as BPF_REG_AX to
> > +			 * materialize the target address. This would clobber AX
> > +			 * and break timed may_goto semantics.
> > +			 *
> > +			 * Emit a minimal indirect call sequence here using a temp
> > +			 * register and skip the normal post-call return-value move.
> > +			 */
> > +
> > +			if (func_addr == (u64)arch_bpf_timed_may_goto) {
> > +				ret = 0;
> > +				if (!IS_ENABLED(CONFIG_PPC_KERNEL_PCREL))
> > +					ret = bpf_jit_emit_func_call(image, ctx, func_addr,
> > +								     tmp1_reg);
> > +
> > +				if (ret || IS_ENABLED(CONFIG_PPC_KERNEL_PCREL)) {
> > +					PPC_LI_ADDR(tmp1_reg, func_addr);
> > +					EMIT(PPC_RAW_MTCTR(tmp1_reg));
> > +					EMIT(PPC_RAW_BCTRL());
> > +				}
> > +
> > +				break;
> > +			}
> > +
> >   			/* Take care of powerpc ABI requirements before kfunc call */
> >   			if (insn[i].src_reg == BPF_PSEUDO_KFUNC_CALL) {
> >   				if (prepare_for_kfunc_call(fp, image, ctx, &insn[i]))
> > diff --git a/arch/powerpc/net/bpf_timed_may_goto.S b/arch/powerpc/net/bpf_timed_may_goto.S
> > new file mode 100644
> > index 000000000000..6fd8b1c9f4ac
> > --- /dev/null
> > +++ b/arch/powerpc/net/bpf_timed_may_goto.S
> > @@ -0,0 +1,57 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/* Copyright (c) 2025 IBM Corporation, Saket Kumar Bhaskar <[email-masked]> */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/ppc_asm.h>
> > +
> > +/*
> > + * arch_bpf_timed_may_goto() trampoline for powerpc64
> > + *
> > + * Custom BPF convention (verifier/JIT):
> > + *	- input:  stack offset in BPF_REG_AX (r12)
> > + *	- output: updated count in BPF_REG_AX (r12)
> > + *
> > + * Call bpf_check_timed_may_goto(ptr) with normal powerpc64 ABI:
> > + * 	- r3 = ptr, return in r3
> > + *
> > + * Preserve BPF regs R0-R5 (mapping: r8, r3-r7).
> > + */
> > +
> > +SYM_FUNC_START(arch_bpf_timed_may_goto)
> > +	/* Prologue: save LR, allocate frame */
> > +	mflr    r0
> > +	std     r0, 16(r1)
> > +	stdu    r1, -112(r1)
> > +
> > +	/* Save BPF registers R0 - R5 (r8, r3-r7) */
> > +	std     r3, 32(r1)
> > +	std     r4, 40(r1)
> > +	std     r5, 48(r1)
> > +	std     r6, 56(r1)
> > +	std     r7, 64(r1)
> > +	std     r8, 72(r1)
> > +
> > +	/*
> > +	 * r3 = BPF_REG_FP + BPF_REG_AX
> > +	 * BPF_REG_FP is r31; BPF_REG_AX is r12 (stack offset in bytes).
> > +	 */
> > +	add     r3, r31, r12
> > +	bl      bpf_check_timed_may_goto
> > +
> > +	/* Put return value back into AX */
> > +	mr      r12, r3
> > +
> > +	/* Restore BPF registers R0 - R5 (r8, r3-r7) */
> > +	ld      r3, 32(r1)
> > +	ld      r4, 40(r1)
> > +	ld      r5, 48(r1)
> > +	ld      r6, 56(r1)
> > +	ld      r7, 64(r1)
> > +	ld      r8, 72(r1)
> > +
> > +	/* Epilogue: pop frame, restore LR, return */
> > +	addi    r1, r1, 112
> > +	ld      r0, 16(r1)
> > +	mtlr    r0
> > +	blr
> > +SYM_FUNC_END(arch_bpf_timed_may_goto)
>