2024-05-05 20:17:18

by Puranjay Mohan

Subject: [PATCH bpf] riscv, bpf: make some atomic operations fully ordered

The BPF atomic operations with the BPF_FETCH modifier along with
BPF_XCHG and BPF_CMPXCHG are fully ordered but the RISC-V JIT implements
all atomic operations except BPF_CMPXCHG with relaxed ordering.

Section 8.1 of the "The RISC-V Instruction Set Manual Volume I:
Unprivileged ISA" [1], titled, "Specifying Ordering of Atomic
Instructions" says:

| To provide more efficient support for release consistency [5], each
| atomic instruction has two bits, aq and rl, used to specify additional
| memory ordering constraints as viewed by other RISC-V harts.

and

| If only the aq bit is set, the atomic memory operation is treated as
| an acquire access.
| If only the rl bit is set, the atomic memory operation is treated as a
| release access.
|
| If both the aq and rl bits are set, the atomic memory operation is
| sequentially consistent.

Fix this by setting both aq and rl bits as 1 for operations with
BPF_FETCH and BPF_XCHG.

[1] https://riscv.org/wp-content/uploads/2017/05/riscv-spec-v2.2.pdf

Fixes: dd642ccb45ec ("riscv, bpf: Implement more atomic operations for RV64")
Signed-off-by: Puranjay Mohan <[email protected]>
---
arch/riscv/net/bpf_jit_comp64.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index ec9d692838fc..fb5d1950042b 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -498,33 +498,33 @@ static void emit_atomic(u8 rd, u8 rs, s16 off, s32 imm, bool is64,
break;
/* src_reg = atomic_fetch_<op>(dst_reg + off16, src_reg) */
case BPF_ADD | BPF_FETCH:
- emit(is64 ? rv_amoadd_d(rs, rs, rd, 0, 0) :
- rv_amoadd_w(rs, rs, rd, 0, 0), ctx);
+ emit(is64 ? rv_amoadd_d(rs, rs, rd, 1, 1) :
+ rv_amoadd_w(rs, rs, rd, 1, 1), ctx);
if (!is64)
emit_zextw(rs, rs, ctx);
break;
case BPF_AND | BPF_FETCH:
- emit(is64 ? rv_amoand_d(rs, rs, rd, 0, 0) :
- rv_amoand_w(rs, rs, rd, 0, 0), ctx);
+ emit(is64 ? rv_amoand_d(rs, rs, rd, 1, 1) :
+ rv_amoand_w(rs, rs, rd, 1, 1), ctx);
if (!is64)
emit_zextw(rs, rs, ctx);
break;
case BPF_OR | BPF_FETCH:
- emit(is64 ? rv_amoor_d(rs, rs, rd, 0, 0) :
- rv_amoor_w(rs, rs, rd, 0, 0), ctx);
+ emit(is64 ? rv_amoor_d(rs, rs, rd, 1, 1) :
+ rv_amoor_w(rs, rs, rd, 1, 1), ctx);
if (!is64)
emit_zextw(rs, rs, ctx);
break;
case BPF_XOR | BPF_FETCH:
- emit(is64 ? rv_amoxor_d(rs, rs, rd, 0, 0) :
- rv_amoxor_w(rs, rs, rd, 0, 0), ctx);
+ emit(is64 ? rv_amoxor_d(rs, rs, rd, 1, 1) :
+ rv_amoxor_w(rs, rs, rd, 1, 1), ctx);
if (!is64)
emit_zextw(rs, rs, ctx);
break;
/* src_reg = atomic_xchg(dst_reg + off16, src_reg); */
case BPF_XCHG:
- emit(is64 ? rv_amoswap_d(rs, rs, rd, 0, 0) :
- rv_amoswap_w(rs, rs, rd, 0, 0), ctx);
+ emit(is64 ? rv_amoswap_d(rs, rs, rd, 1, 1) :
+ rv_amoswap_w(rs, rs, rd, 1, 1), ctx);
if (!is64)
emit_zextw(rs, rs, ctx);
break;
--
2.40.1

2024-05-13 00:00:55

by patchwork-bot+netdevbpf

Subject: Re: [PATCH bpf] riscv, bpf: make some atomic operations fully ordered

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[email protected]>:

On Sun, 5 May 2024 20:16:33 +0000 you wrote:
> The BPF atomic operations with the BPF_FETCH modifier along with
> BPF_XCHG and BPF_CMPXCHG are fully ordered but the RISC-V JIT implements
> all atomic operations except BPF_CMPXCHG with relaxed ordering.
>
> Section 8.1 of the "The RISC-V Instruction Set Manual Volume I:
> Unprivileged ISA" [1], titled, "Specifying Ordering of Atomic
> Instructions" says:
>
> [...]

Here is the summary with links:
- [bpf] riscv, bpf: make some atomic operations fully ordered
https://git.kernel.org/bpf/bpf-next/c/20a759df3bba

You are awesome, thank you!
--
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