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From: Paul Burton <>
To: Alexander Lobakin <>
Cc: Paul Burton <>,
	Hassan Naveed <>,
	Ralf Baechle <>,
	James Hogan <>,
	Alexei Starovoitov <>,
	Daniel Borkmann <>,
	Martin KaFai Lau <>, Song Liu <>,
	Yonghong Song <>, Andrii Nakryiko <>,,,,
Subject: Re: MIPS eBPF JIT support on pre-32R2
Date: Thu, 5 Dec 2019 10:44:50 -0800	[thread overview]
Message-ID: <20191205184450.lbrkenmursz4zpdm@lantea.localdomain> (raw)
In-Reply-To: <>

Hi Alexander,

On Thu, Dec 05, 2019 at 03:45:27PM +0300, Alexander Lobakin wrote:
> 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?

No, they don't; the eBPF JIT makes unconditional use of at least the
(d)ins & (d)ext instructions which were added in MIPSr2, so it would
result in reserved instruction exceptions & panics if enabled on
pre-MIPSr2 CPUs.

> 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:
> should be changed to something like:
> (and then the mentioned 'if' condition would become redundant)

Good spot; I agree entirely, this dependency should be reflected in

> 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).

That would be great, thanks!

One thing to note is that I hope we'll restore the cBPF JIT with this

The cBPF JIT looks like it should work on older pre-MIPSr2 CPUs, so the
only way this is relevant is that your patch might have a minor
conflict. But I thought I'd mention it anyway :)


  parent reply	other threads:[~2019-12-05 18:44 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-05 12:45 Alexander Lobakin
2019-12-05 13:04 ` Daniel Borkmann
2019-12-05 18:44 ` Paul Burton [this message]
2019-12-06  7:20   ` Alexander Lobakin
2019-12-06  8:11     ` Alexander Lobakin

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