[PATCH bpf-next v2 2/3] riscv, bpf: Add support for BPF exceptions
Pu Lehui
pulehui at huawei.com
Sun Jun 28 20:01:58 PDT 2026
On 2026/6/29 1:50, Björn Töpel wrote:
> Varun R Mallya <varunrmallya at gmail.com> writes:
>
>> Add the JIT support required for BPF exceptions (bpf_throw()) on riscv64.
>>
>> Two kinds of program need special prologue/epilogue handling:
>>
>> - A program acting as an exception boundary must save the full set of
>> riscv callee-saved GP registers (ra, s0-s11), not just the ones it
>> happens to clobber, so that the exception callback can restore the
>> state that was live at the boundary. ra and fp are stored first so
>> the saved ra/fp pair forms a valid stackframe record for the
>> unwinder.
>>
>> - The exception callback (exception_cb) does not allocate its own
>> frame. It reuses the boundary program's frame, whose frame pointer
>> is passed in a2, by setting SP = FP - stack_adjust. This lines the
>> epilogue's loads up with the registers the boundary saved, so both
>> paths restore the same order.
>>
>> Wire up bpf_jit_support to be true only when CONFIG_FRAME_POINTER is
>> enabled.
>>
>> Signed-off-by: Varun R Mallya <varunrmallya at gmail.com>
>> Reviewed-by: Pu Lehui <pulehui at huawei.com>
not yet Reviewed-by
>> ---
>> arch/riscv/net/bpf_jit_comp64.c | 115 ++++++++++++++++++++++++++++----
>> 1 file changed, 102 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index c03c1de16b79..c6e2f852e854 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -56,6 +56,30 @@ static const int pt_regmap[] = {
>> [RV_REG_T0] = offsetof(struct pt_regs, t0),
>> };
>>
>> +/*
>> + * Full set of RISC-V callee-saved GP registers (ra, s0-s11) saved by a program
>> + * acting as an exception boundary, in the order they are stored on the stack.
>> + * RA and FP come first so the saved ra/fp pair forms a valid stackframe record
>> + * at [FP-8]/[FP-16] for the unwinder. The exception callback reuses the
>> + * boundary program's frame and restores this same set in its epilogue, so both
>> + * paths must agree on the contents and ordering of this list.
>> + */
>
> nit: Please use netdev style multi-line comments (/* Blah, vs /*\n)
>
>> +static const int rv_exception_csave_regs[] = {
>> + RV_REG_RA,
>> + RV_REG_FP,
>> + RV_REG_S1,
>> + RV_REG_S2,
>> + RV_REG_S3,
>> + RV_REG_S4,
>> + RV_REG_S5,
>> + RV_REG_S6,
>> + RV_REG_S7,
>> + RV_REG_S8,
>> + RV_REG_S9,
>> + RV_REG_S10,
>> + RV_REG_S11,
>> +};
>> +
>> enum {
>> RV_CTX_F_SEEN_TAIL_CALL = 0,
>> RV_CTX_F_SEEN_CALL = RV_REG_RA,
>> @@ -231,6 +255,22 @@ static void emit_imm(u8 rd, s64 val, struct rv_jit_context *ctx)
>> static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>> {
>> int stack_adjust = ctx->stack_size, store_offset = stack_adjust - 8;
>> + struct bpf_prog_aux *aux = ctx->prog->aux;
>> + int i;
>> +
>> + if (aux->exception_boundary || aux->exception_cb) {
>> + /*
>
> nit: Comment again.
>
>> + * An exception boundary saved the full callee-saved register
>> + * set and the exception callback restores it from the boundary's
>> + * frame. Both restore the same fixed set, in the same order it
>> + * was stored by bpf_jit_build_prologue().
>> + */
>> + for (i = 0; i < ARRAY_SIZE(rv_exception_csave_regs); i++) {
>> + emit_ld(rv_exception_csave_regs[i], store_offset, RV_REG_SP, ctx);
>> + store_offset -= 8;
>> + }
>> + goto epilogue_tail;
>> + }
>>
>> if (seen_reg(RV_REG_RA, ctx)) {
>> emit_ld(RV_REG_RA, store_offset, RV_REG_SP, ctx);
>> @@ -267,6 +307,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>> store_offset -= 8;
>> }
>>
>> +epilogue_tail:
>> emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx);
>> /* Set return value. */
>> if (!is_tail_call)
>> @@ -2002,11 +2043,61 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>> void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
>> {
>> int i, stack_adjust = 0, store_offset, bpf_stack_adjust;
>> + struct bpf_prog_aux *aux = ctx->prog->aux;
>>
>> bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, STACK_ALIGN);
>> if (bpf_stack_adjust)
>> mark_fp(ctx);
>>
>> + /* emit kcfi type preamble immediately before the first insn */
>> + emit_kcfi(is_subprog ? cfi_bpf_subprog_hash : cfi_bpf_hash, ctx);
>> +
>> + /* nops reserved for auipc+jalr pair */
>> + for (i = 0; i < RV_FENTRY_NINSNS; i++)
>> + emit(rv_nop(), ctx);
>> +
>> + /* First instruction is always setting the tail-call-counter
>> + * (TCC) register. This instruction is skipped for tail calls.
>> + * Force using a 4-byte (non-compressed) instruction.
>> + */
>> + emit(rv_addi(RV_REG_TCC, RV_REG_ZERO, MAX_TAIL_CALL_CNT), ctx);
>> +
>> + if (aux->exception_boundary || aux->exception_cb) {
>> + /*
>> + * A program acting as an exception boundary saves the full set
>> + * of riscv callee saved registers (ra, s0-s11).
>> + */
>> + stack_adjust = round_up(ARRAY_SIZE(rv_exception_csave_regs) * 8,
>> + STACK_ALIGN);
>> + stack_adjust += bpf_stack_adjust;
>> + store_offset = stack_adjust - 8;
>> +
>> + if (!aux->exception_cb && aux->exception_cb) {
A nit, `if (!aux->exception_cb)` is enough, as aux->exception_cb and
aux->exception_cb progs are independent.
>> + /*
>> + * Boundary program: allocate the frame and save the
>> + * full callee-saved set, capturing the caller's values.
>> + */
>> + emit_addi(RV_REG_SP, RV_REG_SP, -stack_adjust, ctx);
>> + for (i = 0; i < ARRAY_SIZE(rv_exception_csave_regs); i++) {
>> + emit_sd(RV_REG_SP, store_offset,
>> + rv_exception_csave_regs[i], ctx);
>> + store_offset -= 8;
>> + }
>> + emit_addi(RV_REG_FP, RV_REG_SP, stack_adjust, ctx);
>> + } else {
>> + /*
>> + * Exception callback, reuse the boundary program's
>> + * frame, whose frame pointer is passed in a2. Setting
>> + * SP = FP - stack_adjust lines the epilogue's loads up
>> + * with the registers the boundary saved.
>> + */
>> + emit_mv(RV_REG_FP, RV_REG_A2, ctx);
>> + emit_addi(RV_REG_SP, RV_REG_FP, -stack_adjust, ctx);
>> + }
>> +
>> + goto tail_setup;
>> + }
>> +
>
> This function is getting large... I tend do forget details, so having it
> in smaller helpers would be good. Let's try to refactor a bit.
>
> The special case is really just:
> * exception boundary: allocate frame and save full layout
> * exception_cb: reuse boundary FP from a2 and derive SP from it
> * both: restore the same fixed layout
>
> So helpers like is_exception_prog(), exception_stack_adjust(),
> emit_exception_boundary_prologue(), emit_exception_cb_prologue(), and
> emit_exception_restore() would make the prologue/epilogue changes much
> easier to review and also reduce churn in bpf_jit_build_prologue().
>
>> if (seen_reg(RV_REG_RA, ctx))
>> stack_adjust += 8;
>> stack_adjust += 8; /* RV_REG_FP */
>> @@ -2030,19 +2121,6 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
>>
>> store_offset = stack_adjust - 8;
>>
>> - /* emit kcfi type preamble immediately before the first insn */
>> - emit_kcfi(is_subprog ? cfi_bpf_subprog_hash : cfi_bpf_hash, ctx);
>> -
>> - /* nops reserved for auipc+jalr pair */
>> - for (i = 0; i < RV_FENTRY_NINSNS; i++)
>> - emit(rv_nop(), ctx);
>> -
>> - /* First instruction is always setting the tail-call-counter
>> - * (TCC) register. This instruction is skipped for tail calls.
>> - * Force using a 4-byte (non-compressed) instruction.
>> - */
>> - emit(rv_addi(RV_REG_TCC, RV_REG_ZERO, MAX_TAIL_CALL_CNT), ctx);
>> -
>> emit_addi(RV_REG_SP, RV_REG_SP, -stack_adjust, ctx);
>>
>> if (seen_reg(RV_REG_RA, ctx)) {
>> @@ -2082,6 +2160,7 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx, bool is_subprog)
>>
>> emit_addi(RV_REG_FP, RV_REG_SP, stack_adjust, ctx);
>>
>> +tail_setup:
>
> Hmm, thinking more about it, maybe folding the "normal_prologue()" in as
> pre-commit, so we can avoid more gotos.
>
> | if (is_exception_prog(aux))
> | emit_exception_restore(ctx, stack_adjust);
> | else
> | emit_normal_restore(ctx, stack_adjust);
agree this too
> |
> | emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx);
> | ...
>
> or smth.
>
>> if (bpf_stack_adjust) emit_addi(RV_REG_S5, RV_REG_SP,
>> bpf_stack_adjust, ctx);
>>
>> @@ -2157,3 +2236,13 @@ bool bpf_jit_supports_fsession(void)
>> {
>> return true;
>> }
>> +
>> +bool bpf_jit_supports_exceptions(void)
>> +{
>> + /*
>> + * bpf_throw() unwinds by walking the frame-pointer chain from inside
>> + * the kernel back into the BPF frames (see arch_bpf_stack_walk()), so
>> + * exceptions require the frame-pointer unwinder to be enabled.
>> + */
>
> nit: Comments...
>
>
> Thanks,
> Björn
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