Re: MIPS eBPF JIT support on pre-32R2

[Alexander Lobakin]

Paul Burton wrote 05.12.2019 21:44:
> Hi Alexander,

Hi Paul!

> 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:
> >
> > 	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)
>
> Good spot; I agree entirely, this dependency should be reflected in
> Kconfig.
>
> > 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!

Great, I'll send it in about ~2-3 hours.

> One thing to note is that I hope we'll restore the cBPF JIT with this
> patch:
>
> https://lore.kernel.org/linux-mips/20191205182318.2761605-1-paulburton@xxxxxxxxxx/T/#u
>
> 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 :)

Yes, I thought about this too. If pre-32R2 CPUs don't support our eBPF
JIT, we'd better restore cBPF for them, so they could speed-up at least
"classic" instructions. Glad you've decided to do that.

> Thanks,
>     Paul

Regards,
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