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Date: Mon, 7 Dec 2020 17:44:12 -0800
From: Yonghong Song <yhs@...com>
To: Brendan Jackman <jackmanb@...gle.com>, <bpf@...r.kernel.org>
CC: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
KP Singh <kpsingh@...omium.org>,
Florent Revest <revest@...omium.org>,
<linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH bpf-next v4 07/11] bpf: Add instructions for
atomic_[cmp]xchg
On 12/7/20 8:07 AM, Brendan Jackman wrote:
> This adds two atomic opcodes, both of which include the BPF_FETCH
> flag. XCHG without the BPF_FETCH flag would naturally encode
> atomic_set. This is not supported because it would be of limited
> value to userspace (it doesn't imply any barriers). CMPXCHG without
> BPF_FETCH woulud be an atomic compare-and-write. We don't have such
> an operation in the kernel so it isn't provided to BPF either.
>
> There are two significant design decisions made for the CMPXCHG
> instruction:
>
> - To solve the issue that this operation fundamentally has 3
> operands, but we only have two register fields. Therefore the
> operand we compare against (the kernel's API calls it 'old') is
> hard-coded to be R0. x86 has similar design (and A64 doesn't
> have this problem).
>
> A potential alternative might be to encode the other operand's
> register number in the immediate field.
>
> - The kernel's atomic_cmpxchg returns the old value, while the C11
> userspace APIs return a boolean indicating the comparison
> result. Which should BPF do? A64 returns the old value. x86 returns
> the old value in the hard-coded register (and also sets a
> flag). That means return-old-value is easier to JIT.
>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
> arch/x86/net/bpf_jit_comp.c | 8 ++++++++
> include/linux/filter.h | 22 ++++++++++++++++++++++
> include/uapi/linux/bpf.h | 4 +++-
> kernel/bpf/core.c | 20 ++++++++++++++++++++
> kernel/bpf/disasm.c | 15 +++++++++++++++
> kernel/bpf/verifier.c | 19 +++++++++++++++++--
> tools/include/linux/filter.h | 22 ++++++++++++++++++++++
> tools/include/uapi/linux/bpf.h | 4 +++-
> 8 files changed, 110 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index eea7d8b0bb12..308241187582 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -815,6 +815,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
> /* src_reg = atomic_fetch_add(dst_reg + off, src_reg); */
> EMIT2(0x0F, 0xC1);
> break;
> + case BPF_XCHG:
> + /* src_reg = atomic_xchg(dst_reg + off, src_reg); */
> + EMIT1(0x87);
> + break;
> + case BPF_CMPXCHG:
> + /* r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg); */
> + EMIT2(0x0F, 0xB1);
> + break;
> default:
> pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op);
> return -EFAULT;
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index b5258bca10d2..e1e1fc946a7c 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -265,6 +265,8 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
> *
> * BPF_ADD *(uint *) (dst_reg + off16) += src_reg
> * BPF_ADD | BPF_FETCH src_reg = atomic_fetch_add(dst_reg + off16, src_reg);
> + * BPF_XCHG src_reg = atomic_xchg(dst_reg + off16, src_reg)
> + * BPF_CMPXCHG r0 = atomic_cmpxchg(dst_reg + off16, r0, src_reg)
> */
>
> #define BPF_ATOMIC64(OP, DST, SRC, OFF) \
> @@ -293,6 +295,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
> .off = OFF, \
> .imm = BPF_ADD })
>
> +/* Atomic exchange, src_reg = atomic_xchg(dst_reg + off, src_reg) */
> +
> +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \
> + ((struct bpf_insn) { \
> + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> + .dst_reg = DST, \
> + .src_reg = SRC, \
> + .off = OFF, \
> + .imm = BPF_XCHG })
> +
> +/* Atomic compare-exchange, r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg) */
> +
> +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \
> + ((struct bpf_insn) { \
> + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
> + .dst_reg = DST, \
> + .src_reg = SRC, \
> + .off = OFF, \
> + .imm = BPF_CMPXCHG })
Define BPF_ATOMIC_{XCHG, CMPXCHG} based on BPF_ATOMIC macro?
> +
> /* Memory store, *(uint *) (dst_reg + off16) = imm32 */
>
> #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index d5389119291e..b733af50a5b9 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -45,7 +45,9 @@
> #define BPF_EXIT 0x90 /* function return */
>
> /* atomic op type fields (stored in immediate) */
> -#define BPF_FETCH 0x01 /* fetch previous value into src reg */
> +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
> +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
> +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
>
> /* Register numbers */
> enum {
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 61e93eb7d363..28f960bc2e30 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1630,6 +1630,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> (u32) SRC,
> (atomic_t *)(unsigned long) (DST + insn->off));
> break;
> + case BPF_XCHG:
> + SRC = (u32) atomic_xchg(
> + (atomic_t *)(unsigned long) (DST + insn->off),
> + (u32) SRC);
> + break;
> + case BPF_CMPXCHG:
> + BPF_R0 = (u32) atomic_cmpxchg(
> + (atomic_t *)(unsigned long) (DST + insn->off),
> + (u32) BPF_R0, (u32) SRC);
> + break;
> default:
> goto default_label;
> }
[...]
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