From: "Björn Töpel" <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
Date: Wed, 30 Nov 2022 17:04:04 +0100 [thread overview]
Message-ID: <87a648ig3v.fsf@all.your.base.are.belong.to.us> (raw)
In-Reply-To: <87h6yg1xlo.fsf@all.your.base.are.belong.to.us>
Björn Töpel <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 = 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
next prev parent reply other threads:[~2022-11-30 16:04 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-30 3:38 Pu Lehui
2022-11-30 11:38 ` Björn Töpel
2022-11-30 16:04 ` Björn Töpel [this message]
2022-12-02 9:58 ` Pu Lehui
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