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Subject: Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for
 atomic[64]_[fetch_]sub
To:     Brendan Jackman <jackmanb@google.com>, <bpf@vger.kernel.org>
CC:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>
References: <20201127175738.1085417-1-jackmanb@google.com>
 <20201127175738.1085417-11-jackmanb@google.com>
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On 11/27/20 9:57 AM, Brendan Jackman wrote:
> Including only interpreter and x86 JIT support.
> 
> x86 doesn't provide an atomic exchange-and-subtract instruction that
> could be used for BPF_SUB | BPF_FETCH, however we can just emit a NEG
> followed by an XADD to get the same effect.
> 
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
>   arch/x86/net/bpf_jit_comp.c  | 16 ++++++++++++++--
>   include/linux/filter.h       | 20 ++++++++++++++++++++
>   kernel/bpf/core.c            |  1 +
>   kernel/bpf/disasm.c          | 16 ++++++++++++----
>   kernel/bpf/verifier.c        |  2 ++
>   tools/include/linux/filter.h | 20 ++++++++++++++++++++
>   6 files changed, 69 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 7431b2937157..a8a9fab13fcf 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -823,6 +823,7 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
>   
>   	/* emit opcode */
>   	switch (atomic_op) {
> +	case BPF_SUB:
>   	case BPF_ADD:
>   		/* lock *(u32/u64*)(dst_reg + off) <op>= src_reg */
>   		EMIT1(simple_alu_opcodes[atomic_op]);
> @@ -1306,8 +1307,19 @@ st:			if (is_imm8(insn->off))
>   
>   		case BPF_STX | BPF_ATOMIC | BPF_W:
>   		case BPF_STX | BPF_ATOMIC | BPF_DW:
> -			err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
> -					  insn->off, BPF_SIZE(insn->code));
> +			if (insn->imm == (BPF_SUB | BPF_FETCH)) {
> +				/*
> +				 * x86 doesn't have an XSUB insn, so we negate
> +				 * and XADD instead.
> +				 */
> +				emit_neg(&prog, src_reg, BPF_SIZE(insn->code) == BPF_DW);
> +				err = emit_atomic(&prog, BPF_ADD | BPF_FETCH,
> +						  dst_reg, src_reg, insn->off,
> +						  BPF_SIZE(insn->code));
> +			} else {
> +				err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
> +						  insn->off, BPF_SIZE(insn->code));
> +			}
>   			if (err)
>   				return err;
>   			break;
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 6186280715ed..a20a3a536bf5 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
>   		.off   = OFF,					\
>   		.imm   = BPF_ADD | BPF_FETCH })
>   
> +/* Atomic memory sub, *(uint *)(dst_reg + off16) -= src_reg */
> +
> +#define BPF_ATOMIC_SUB(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_SUB })

Currently, llvm does not support XSUB, should we support it in llvm?
At source code, as implemented in JIT, user can just do a negate
followed by xadd.

> +
> +/* Atomic memory sub with fetch, src_reg = atomic_fetch_sub(*(dst_reg + off), src_reg); */
> +
> +#define BPF_ATOMIC_FETCH_SUB(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_SUB | BPF_FETCH })
> +
>   /* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
>   
>   #define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF)			\
[...]