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From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     Pu Lehui <pulehui@huaweicloud.com>, bpf@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Pu Lehui <pulehui@huawei.com>,
        Pu Lehui <pulehui@huaweicloud.com>
Subject: Re: [PATCH bpf] riscv, bpf: Emit fixed-length imm64 for
 BPF_PSEUDO_FUNC
In-Reply-To: <87h6yg1xlo.fsf@all.your.base.are.belong.to.us>
References: <20221130033806.2967822-1-pulehui@huaweicloud.com>
 <87h6yg1xlo.fsf@all.your.base.are.belong.to.us>
Date:   Wed, 30 Nov 2022 17:04:04 +0100
Message-ID: <87a648ig3v.fsf@all.your.base.are.belong.to.us>
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Bj=C3=B6rn T=C3=B6pel <bjorn@kernel.org> writes:

> The range of valid address for RV64 (sv39 to sv57) are
> 0xffffffff00000000 to 0xffffffffffffffff, so I think we can do better
> than 6 insn, no? My gut feeling (I need to tinker a bit) is that 4
> should be sufficient.

Ok, thinking a bit more about it; A proper address will at have at least
2B alignment, so that means that we can construct an address with lui,
addi, and slli (31 bits).

u64 addr =3D 0xffffffff12345678;

u32 (imm |=3D 0xffffffffUL) >> 1;
u32 upper =3D (imm + (1 << 11)) >> 12;
u32 lower =3D imm & 0xfff;
u32 rupper =3D upper | 0x80000; // for sign extend

NB! Make sure it's !C insn, and emit:

lui xx, rupper
addi xx, xx, lower
slli xx, xx, 1

Now we'll have fixed three insns. WDYT?


Bj=C3=B6rn