linux-kernel - Re: [PATCH bpf] riscv, bpf: Emit fixed-length imm64 for BPF_PSEUDO

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Date:   Wed, 30 Nov 2022 17:04:04 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Pu Lehui <pulehui@...wei.com>,
        Pu Lehui <pulehui@...weicloud.com>
Subject: Re: [PATCH bpf] riscv, bpf: Emit fixed-length imm64 for
 BPF_PSEUDO_FUNC

Björn Töpel <bjorn@...nel.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 = 0xffffffff12345678;

u32 (imm |= 0xffffffffUL) >> 1;
u32 upper = (imm + (1 << 11)) >> 12;
u32 lower = imm & 0xfff;
u32 rupper = 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?


Björn