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Message-ID: <87a648ig3v.fsf@all.your.base.are.belong.to.us>
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
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