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Sat, 28 Nov 2020 05:35:10 +0000 Subject: Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub To: Brendan Jackman , CC: Alexei Starovoitov , Daniel Borkmann , KP Singh , Florent Revest , , Jann Horn References: <20201127175738.1085417-1-jackmanb@google.com> <20201127175738.1085417-11-jackmanb@google.com> From: Yonghong Song Message-ID: <0fd52966-24b2-c50c-4f23-93428d8993c4@fb.com> Date: Fri, 27 Nov 2020 21:35:07 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 In-Reply-To: <20201127175738.1085417-11-jackmanb@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:fca2] X-ClientProxiedBy: MWHPR10CA0001.namprd10.prod.outlook.com (2603:10b6:301::11) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21c8::102b] (2620:10d:c090:400::5:fca2) by MWHPR10CA0001.namprd10.prod.outlook.com (2603:10b6:301::11) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3611.20 via Frontend Transport; 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> @@ -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) \ [...]