[PATCH bpf-next v3 1/2] bpf, riscv: Add BPF stack arguments support for RV64 JIT
Pu Lehui
pulehui at huawei.com
Sun Aug 16 18:52:20 PDT 2026
Hi Feng Jiang,
On 2026/8/14 9:29, Feng Jiang wrote:
> Add bpf_jit_supports_stack_args() for the RV64 JIT so BPF subprograms
> and kfuncs can receive more than 5 arguments via the stack
> (BPF_REG_PARAMS / r11).
>
> For BPF-to-BPF calls the caller writes outgoing arguments at the
> bottom of its frame (SP-relative). The callee reads them with
> FP-relative loads. Its FP is set to the caller SP in the prologue,
> so the offsets match.
>
> The RISC-V ABI puts arguments 6-8 in A5-A7 and arguments 9+ at
> SP+0. Before each kfunc call, load arguments 6-8 from the outgoing
> area into A5-A7 and copy any remaining arguments down so argument 9
> lands at SP+0.
bpf2bpf calls pass extra arguments entirely on the stack, whereas kfuncs
pass them via a mix of registers and the stack. This inconsistency not
only wastes stack slots in kfunc scenarios but is also error-prone and
easy to overlook during maintenance.
To make argument passing more straightforward and consistent, I think we
can align it with the standard riscv calling convention: use a0-a7 for
the first 8 arguments and pass any remaining arguments on the stack.
To support this, the current bpf-to-rv register mapping needs to be revised:
T0 -> BPF_REG_0 (ftrace use T0 too, but it might not be an issue / needs
verification)
T6 -> TCC
T7 -> BPF_REG_AX (rarely used, so let's move to t7)
Note that this remapping affects multiple places and requires careful
auditing.
>
> A5 (BPF_REG_0), A6 (TCC) and A7 are safe to clobber here: R0 is not
> live before a call, TCC is backed up on the stack, and A7 is unused
> by the JIT.
>
> Limit the existing kfunc sign-extension loop to MAX_BPF_FUNC_REG_ARGS
> iterations; otherwise idx >= 5 resolves to S1-S5 (BPF_R6-FP) and
> corrupts callee-saved registers.
>
> Signed-off-by: Feng Jiang <jiangfeng at kylinos.cn>
> ---
> arch/riscv/net/bpf_jit.h | 1 +
> arch/riscv/net/bpf_jit_comp64.c | 79 +++++++++++++++++++++++++++++++++++++++--
> arch/riscv/net/bpf_jit_core.c | 4 +++
> 3 files changed, 81 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index 419b9d795f2a..9eb4e149505d 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -82,6 +82,7 @@ struct rv_jit_context {
> unsigned long flags;
> int stack_size;
> int tcc_offset;
> + u16 stack_arg_size;
> u64 arena_vm_start;
> u64 user_vm_start;
> };
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 6b9972b07c1b..58cb3e5ff6b4 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1815,18 +1815,43 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>
> if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {
> const struct btf_func_model *fm;
> - int idx;
> + int idx, nargs;
>
> fm = bpf_jit_find_kfunc_model(ctx->prog, insn);
> if (!fm)
> return -EINVAL;
>
> - for (idx = 0; idx < fm->nr_args; idx++) {
> + nargs = min_t(int, fm->nr_args, MAX_BPF_FUNC_REG_ARGS);
> + for (idx = 0; idx < nargs; idx++) {
> u8 reg = bpf_to_rv_reg(BPF_REG_1 + idx, ctx);
>
> if (fm->arg_size[idx] == sizeof(int))
> emit_sextw(reg, reg, ctx);
> }
> +
> + /* BPF stack args -> RISC-V ABI: args 6-8 in A5-A7, 9+ at SP+0 */
> + if (fm->nr_args > MAX_BPF_FUNC_REG_ARGS) {
> + int n_stack = fm->nr_args - MAX_BPF_FUNC_REG_ARGS;
> + int n_reg = min_t(int, n_stack,
> + RV_MAX_REG_ARGS - MAX_BPF_FUNC_REG_ARGS);
> +
> + for (idx = 0; idx < n_reg; idx++) {
> + int sz = fm->arg_size[MAX_BPF_FUNC_REG_ARGS + idx];
> +
> + emit_ld(RV_REG_A5 + idx, idx * 8, RV_REG_SP, ctx);
> + if (sz == sizeof(int))
> + emit_sextw(RV_REG_A5 + idx, RV_REG_A5 + idx, ctx);
> + }
> +
> + for (idx = n_reg; idx < n_stack; idx++) {
> + int sz = fm->arg_size[MAX_BPF_FUNC_REG_ARGS + idx];
> +
> + emit_ld(RV_REG_T1, idx * 8, RV_REG_SP, ctx);
> + if (sz == sizeof(int))
> + emit_sextw(RV_REG_T1, RV_REG_T1, ctx);
> + emit_sd(RV_REG_SP, (idx - n_reg) * 8, RV_REG_T1, ctx);
> + }
> + }
looks a bit complicated, how about the following after redefine regmap:
```
for (idx = 0; idx < fm->nr_args; idx++) {
if (idx < RV_MAX_REG_ARGS) {
u8 reg = RV_REG_A0 + idx;
sign_extend(reg, reg, xxx)
} else {
load from stack to T1
sign_extend
retore from T1 to orig stack slot
}
}
```
> }
>
> /* restore TCC to RV_REG_TCC before bpf2bpf call */
> @@ -1891,6 +1916,21 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> case BPF_LDX | BPF_MEM | BPF_H:
> case BPF_LDX | BPF_MEM | BPF_W:
> case BPF_LDX | BPF_MEM | BPF_DW:
> + if (insn->src_reg == BPF_REG_PARAMS) {
pls use is_stack_arg_ldx/st/stx instead.
> + int idx = off / 8 - 1;
> +
> + if (is_12b_int(idx * 8)) {
> + emit_ldx_insn(rd, idx * 8, RV_REG_FP, BPF_SIZE(code), false, ctx);
> + } else {
> + emit_imm(RV_REG_T1, idx * 8, ctx);
> + emit_add(RV_REG_T1, RV_REG_T1, RV_REG_FP, ctx);
> + emit_ldx_insn(rd, 0, RV_REG_T1, BPF_SIZE(code), false, ctx);
> + }
> + if (BPF_SIZE(code) != BPF_DW && insn_is_zext(&insn[1]))
> + return 1;
> + break;
This piece of code was refactored not long ago. Let's see if we can
merge it into the logic below or fold it into emit_ldx—let's give it a try.
> + }
> + fallthrough;
> case BPF_LDX | BPF_PROBE_MEM | BPF_B:
> case BPF_LDX | BPF_PROBE_MEM | BPF_H:
> case BPF_LDX | BPF_PROBE_MEM | BPF_W:
> @@ -1938,6 +1978,20 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> case BPF_ST | BPF_MEM | BPF_H:
> case BPF_ST | BPF_MEM | BPF_W:
> case BPF_ST | BPF_MEM | BPF_DW:
> + if (insn->dst_reg == BPF_REG_PARAMS) {
> + int idx = -off / 8 - 1;
> +
> + emit_imm(RV_REG_T1, imm, ctx);
> + if (is_12b_int(idx * 8)) {
> + emit_stx_insn(RV_REG_SP, idx * 8, RV_REG_T1, BPF_SIZE(code), ctx);
> + } else {
> + emit_imm(RV_REG_T2, idx * 8, ctx);
> + emit_add(RV_REG_T2, RV_REG_SP, RV_REG_T2, ctx);
> + emit_stx_insn(RV_REG_T2, 0, RV_REG_T1, BPF_SIZE(code), ctx);
> + }
> + break;
> + }
> + fallthrough;
ditto
> /* ST | PROBE_MEM32: *(size *)(dst + RV_REG_ARENA + off) = imm */
> case BPF_ST | BPF_PROBE_MEM32 | BPF_B:
> case BPF_ST | BPF_PROBE_MEM32 | BPF_H:
> @@ -1960,6 +2014,19 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> case BPF_STX | BPF_MEM | BPF_H:
> case BPF_STX | BPF_MEM | BPF_W:
> case BPF_STX | BPF_MEM | BPF_DW:
> + if (insn->dst_reg == BPF_REG_PARAMS) {
> + int idx = -off / 8 - 1;
> +
> + if (is_12b_int(idx * 8)) {
> + emit_stx_insn(RV_REG_SP, idx * 8, rs, BPF_SIZE(code), ctx);
> + } else {
> + emit_imm(RV_REG_T1, idx * 8, ctx);
> + emit_add(RV_REG_T1, RV_REG_SP, RV_REG_T1, ctx);
> + emit_stx_insn(RV_REG_T1, 0, rs, BPF_SIZE(code), ctx);
> + }
> + break;
> + }
> + fallthrough;
ditto
> /* STX | PROBE_MEM32: *(size *)(dst + RV_REG_ARENA + off) = src */
> case BPF_STX | BPF_PROBE_MEM32 | BPF_B:
> case BPF_STX | BPF_PROBE_MEM32 | BPF_H:
> @@ -2036,6 +2103,7 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
>
> stack_adjust = round_up(stack_adjust, STACK_ALIGN);
> stack_adjust += bpf_stack_adjust;
> + stack_adjust += ctx->stack_arg_size;
>
> store_offset = stack_adjust - 8;
>
> @@ -2093,7 +2161,7 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
> emit_addi(RV_REG_FP, RV_REG_SP, stack_adjust, ctx);
>
> if (bpf_stack_adjust)
> - emit_addi(RV_REG_S5, RV_REG_SP, bpf_stack_adjust, ctx);
> + emit_addi(RV_REG_S5, RV_REG_SP, ctx->stack_arg_size + bpf_stack_adjust, ctx);
>
> ctx->stack_size = stack_adjust;
>
> @@ -2171,3 +2239,8 @@ bool bpf_jit_supports_timed_may_goto(void)
> {
> return true;
> }
> +
> +bool bpf_jit_supports_stack_args(void)
> +{
> + return true;
> +}
> diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
> index cbfcd287ea16..844a0f3e0fa9 100644
> --- a/arch/riscv/net/bpf_jit_core.c
> +++ b/arch/riscv/net/bpf_jit_core.c
> @@ -72,6 +72,10 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
> ctx->arena_vm_start = bpf_arena_get_kern_vm_start(prog->aux->arena);
> ctx->user_vm_start = bpf_arena_get_user_vm_start(prog->aux->arena);
> ctx->prog = prog;
> +
> + ctx->stack_arg_size = round_up(bpf_out_stack_arg_cnt(env, prog) *
> + sizeof(u64), STACK_ALIGN);
It's not u64—since every stack slot is aligned to the pointer size
(sizeof(long)), I think using 8 is fine.
> +
> ctx->offset = kvzalloc_objs(int, prog->len);
> if (!ctx->offset)
> goto out_offset;
>
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