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Date:   Thu, 05 Dec 2019 15:45:27 +0300
From:   Alexander Lobakin <alobakin@dlink.ru>
To:     Paul Burton <paul.burton@mips.com>
Cc:     Hassan Naveed <hnaveed@wavecomp.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        James Hogan <jhogan@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>, netdev@vger.kernel.org,
        bpf@vger.kernel.org, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: MIPS eBPF JIT support on pre-32R2
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Hey all,

I'm writing about lines arch/mips/net/ebpf_jit.c:1806-1807:

	if (!prog->jit_requested || MIPS_ISA_REV < 2)
		return prog;

Do pre-32R2 architectures (32R1, maybe even R3000-like) actually support
this eBPF JIT code? If they do, then the condition 'MIPS_ISA_REV < 2'
should be removed as it is always true for them and tells CC to remove
JIT completely.

If they don't support instructions from this JIT, then the line
arch/mips/Kconfig:50:

	select HAVE_EBPF_JIT if (!CPU_MICROMIPS)

should be changed to something like:

	select HAVE_EBPF_JIT if !CPU_MICROMIPS && TARGET_ISA_REV >= 2

(and then the mentioned 'if' condition would become redundant)

At the moment it is possible to build a kernel without both JIT and
interpreter, but with CONFIG_BPF_SYSCALL=y (what should not be allowed
I suppose?) within the following configuration:

- select any pre-32R2 CPU (e.g. CONFIG_CPU_MIPS32_R1);
- enable CONFIG_BPF_JIT (CONFIG_MIPS_EBPF_JIT will be autoselected);
- enable CONFIG_BPF_JIT_ALWAYS_ON (this removes BPF interpreter from
   the system).

I may prepare a proper patch by myself if needed (after clarification).
Thanks.

Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ