Re: [PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions

From: Puranjay Mohan
Date: Mon Mar 25 2024 - 13:56:23 EST

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Björn Töpel <bjorn@xxxxxxxxxx> writes:

> Puranjay Mohan <puranjay12@xxxxxxxxx> writes:
>
>> Add support for [LDX | STX | ST], PROBE_MEM32, [B | H | W | DW]
>> instructions. They are similar to PROBE_MEM instructions with the
>> following differences:
>> - PROBE_MEM32 supports store.
>> - PROBE_MEM32 relies on the verifier to clear upper 32-bit of the
>> src/dst register
>> - PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in S7
>> in the prologue). Due to bpf_arena constructions such S7 + reg +
>> off16 access is guaranteed to be within arena virtual range, so no
>> address check at run-time.
>> - S7 is a free callee-saved register, so it is used to store kern_vm_start
>> - PROBE_MEM32 allows STX and ST. If they fault the store is a nop. When
>> LDX faults the destination register is zeroed.
>>
>> To support these on riscv, we do tmp = S7 + src/dst reg and then use
>> tmp2 as the new src/dst register. This allows us to reuse most of the
>> code for normal [LDX | STX | ST].
>
> Cool to see the RV BPF JIT keeping up with x86 features! ;-) Nice work!

It is my self proclaimed duty to make sure that all 64-bit JITs have
feature parity. :D

>
> A couple of minor comments below.
>
>> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
>> ---
>> arch/riscv/net/bpf_jit.h | 1 +
>> arch/riscv/net/bpf_jit_comp64.c | 193 +++++++++++++++++++++++++++++++-
>> arch/riscv/net/bpf_jit_core.c | 1 +
>> 3 files changed, 192 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
>> index f4b6b3b9edda..8a47da08dd9c 100644
>> --- a/arch/riscv/net/bpf_jit.h
>> +++ b/arch/riscv/net/bpf_jit.h
>> @@ -81,6 +81,7 @@ struct rv_jit_context {
>> int nexentries;
>> unsigned long flags;
>> int stack_size;
>> + u64 arena_vm_start;
>> };
>>
>> /* Convert from ninsns to bytes. */
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index 1adf2f39ce59..0c0588e327af 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -255,6 +255,10 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>> emit_ld(RV_REG_S6, store_offset, RV_REG_SP, ctx);
>> store_offset -= 8;
>> }
>> + if (ctx->arena_vm_start) {
>> + emit_ld(RV_REG_S7, store_offset, RV_REG_SP, ctx);
>> + store_offset -= 8;
>> + }
>>
>> emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx);
>> /* Set return value. */
>> @@ -548,6 +552,7 @@ static void emit_atomic(u8 rd, u8 rs, s16 off, s32 imm, bool is64,
>>
>> #define BPF_FIXUP_OFFSET_MASK GENMASK(26, 0)
>> #define BPF_FIXUP_REG_MASK GENMASK(31, 27)
>> +#define DONT_CLEAR 17 /* RV_REG_A7 unused in pt_regmap */
>
> Hmm, so this is just a a sentinel node, right? Isn't it more robust to
> use, say REG_ZERO which will never be used? Maybe REG_DONT_CLEAR_MARKER
> or smth, so it's obvious how it's used?

Yes, I agree, RV_REG_ZERO would be the best thing to use here.

>
>
>> bool ex_handler_bpf(const struct exception_table_entry *ex,
>> struct pt_regs *regs)
>> @@ -555,7 +560,8 @@ bool ex_handler_bpf(const struct exception_table_entry *ex,
>> off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup);
>> int regs_offset = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup);
>>
>> - *(unsigned long *)((void *)regs + pt_regmap[regs_offset]) = 0;
>> + if (regs_offset != DONT_CLEAR)
>> + *(unsigned long *)((void *)regs + pt_regmap[regs_offset]) = 0;
>> regs->epc = (unsigned long)&ex->fixup - offset;
>>
>> return true;
>> @@ -572,7 +578,8 @@ static int add_exception_handler(const struct bpf_insn *insn,
>> off_t fixup_offset;
>>
>> if (!ctx->insns || !ctx->ro_insns || !ctx->prog->aux->extable ||
>> - (BPF_MODE(insn->code) != BPF_PROBE_MEM && BPF_MODE(insn->code) != BPF_PROBE_MEMSX))
>> + (BPF_MODE(insn->code) != BPF_PROBE_MEM && BPF_MODE(insn->code) != BPF_PROBE_MEMSX &&
>> + BPF_MODE(insn->code) != BPF_PROBE_MEM32))
>> return 0;
>>
>> if (WARN_ON_ONCE(ctx->nexentries >= ctx->prog->aux->num_exentries))
>> @@ -622,6 +629,9 @@ static int add_exception_handler(const struct bpf_insn *insn,
>>
>> ex->insn = ins_offset;
>>
>> + if (BPF_CLASS(insn->code) != BPF_LDX)
>> + dst_reg = DONT_CLEAR;
>> +
>
> Instead of having a side-effect, and passing a dummy dst_reg for the
> probe_mem32, just explicitly add DONT_CLEAR when calling
> add_exception_handler(). It's more obvious to me at least.

Sure, will do that in the next version.

>
>> ex->fixup = FIELD_PREP(BPF_FIXUP_OFFSET_MASK, fixup_offset) |
>> FIELD_PREP(BPF_FIXUP_REG_MASK, dst_reg);
>> ex->type = EX_TYPE_BPF;
>> @@ -1063,7 +1073,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>> BPF_CLASS(insn->code) == BPF_JMP;
>> int s, e, rvoff, ret, i = insn - ctx->prog->insnsi;
>> struct bpf_prog_aux *aux = ctx->prog->aux;
>> - u8 rd = -1, rs = -1, code = insn->code;
>> + u8 rd = -1, rs = -1, code = insn->code, reg_arena_vm_start = RV_REG_S7;
>> s16 off = insn->off;
>> s32 imm = insn->imm;
>>
>> @@ -1539,6 +1549,11 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>> case BPF_LDX | BPF_PROBE_MEMSX | BPF_B:
>> case BPF_LDX | BPF_PROBE_MEMSX | BPF_H:
>> case BPF_LDX | BPF_PROBE_MEMSX | BPF_W:
>> + /* LDX | PROBE_MEM32: dst = *(unsigned size *)(src + S7 + off)*/
>> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_B:
>> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_H:
>> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_W:
>> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_DW:
>> {
>> int insn_len, insns_start;
>> bool sign_ext;
>> @@ -1546,6 +1561,11 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>> sign_ext = BPF_MODE(insn->code) == BPF_MEMSX ||
>> BPF_MODE(insn->code) == BPF_PROBE_MEMSX;
>>
>> + if (BPF_MODE(insn->code) == BPF_PROBE_MEM32) {
>> + emit_add(RV_REG_T2, rs, reg_arena_vm_start, ctx);
>> + rs = RV_REG_T2;
>> + }
>> +
>> switch (BPF_SIZE(code)) {
>> case BPF_B:
>> if (is_12b_int(off)) {
>> @@ -1682,6 +1702,87 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>> emit_sd(RV_REG_T2, 0, RV_REG_T1, ctx);
>> break;
>>
>> + case BPF_ST | BPF_PROBE_MEM32 | BPF_B:
>> + case BPF_ST | BPF_PROBE_MEM32 | BPF_H:
>> + case BPF_ST | BPF_PROBE_MEM32 | BPF_W:
>> + case BPF_ST | BPF_PROBE_MEM32 | BPF_DW:
>> + {
>> + int insn_len, insns_start;
>> +
>> + emit_add(RV_REG_T3, rd, reg_arena_vm_start, ctx);
>> + rd = RV_REG_T3;
>> +
>> + /* Load imm to a register then store it */
>> + emit_imm(RV_REG_T1, imm, ctx);
>> +
>> + switch (BPF_SIZE(code)) {
>> + case BPF_B:
>> + if (is_12b_int(off)) {
>> + insns_start = ctx->ninsns;
>> + emit(rv_sb(rd, off, RV_REG_T1), ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + }
>> +
>> + emit_imm(RV_REG_T2, off, ctx);
>> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
>> + insns_start = ctx->ninsns;
>> + emit(rv_sb(RV_REG_T2, 0, RV_REG_T1), ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> +
>> + break;
>> +
>> + case BPF_H:
>> + if (is_12b_int(off)) {
>> + insns_start = ctx->ninsns;
>> + emit(rv_sh(rd, off, RV_REG_T1), ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + }
>> +
>> + emit_imm(RV_REG_T2, off, ctx);
>> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
>> + insns_start = ctx->ninsns;
>> + emit(rv_sh(RV_REG_T2, 0, RV_REG_T1), ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + case BPF_W:
>> + if (is_12b_int(off)) {
>> + insns_start = ctx->ninsns;
>> + emit_sw(rd, off, RV_REG_T1, ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + }
>> +
>> + emit_imm(RV_REG_T2, off, ctx);
>> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
>> + insns_start = ctx->ninsns;
>> + emit_sw(RV_REG_T2, 0, RV_REG_T1, ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + case BPF_DW:
>> + if (is_12b_int(off)) {
>> + insns_start = ctx->ninsns;
>> + emit_sd(rd, off, RV_REG_T1, ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + }
>> +
>> + emit_imm(RV_REG_T2, off, ctx);
>> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
>> + insns_start = ctx->ninsns;
>> + emit_sd(RV_REG_T2, 0, RV_REG_T1, ctx);
>> + insn_len = ctx->ninsns - insns_start;
>> + break;
>> + }
>
> A lot of similar code, with emit of different sizes. Possible to move
> move out to a function, and wrap the emits? The main loop is hard read
> already!

I thought about this as well. My plan is to refactor the whole thing in a
seperate patch. I did not do it with this feature as it will cause a lot
of unrelated code churn.

Thanks,
Puranjay

Next message: Jijie Shao: "Re: [PATCH V3 net 1/3] net: hns3: fix index limit to support all queue stats"
Previous message: Kairui Song: "[PATCH v2 4/4] mm/filemap: optimize filemap folio adding"
In reply to: Björn Töpel: "Re: [PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions"
Next in thread: Pu Lehui: "Re: [PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions"
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