[PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions
Pu Lehui
pulehui at huaweicloud.com
Mon Mar 25 20:15:47 PDT 2024
On 2024/3/26 1:15, Puranjay Mohan wrote:
> Björn Töpel <bjorn at kernel.org> writes:
>
>> Puranjay Mohan <puranjay12 at gmail.com> 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 at gmail.com>
>>> ---
>>> 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;
>>> + }
As RV_REG_S7 is only for bpf arena, how about define this register as
bellow, like RV_REG_TCC
#define RV_REG_ARENA RV_REG_S7
>>>
>>> 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.
Yeah, I think we could do that factor out for LDX, ST, STX, while I had
done it before another riscv bpf arena. BUT, looking forword to your
implementation.😄
>
> Thanks,
> Puranjay
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