Re: MIPS eBPF JIT support on pre-32R2

[Alexander Lobakin]

Alexander Lobakin ÐÐÑÐÐ 06.12.2019 10:20:
> 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.

Here we are: [1]
Yep, this will conflict with your patch restoring cBPF for MIPS32.
If you have any questions about my patch or I should change anything in
it, please let me know.
Thanks!

> > 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,
> á á á á á á

[1] 
https://lore.kernel.org/linux-mips/20191206080741.12306-1-alobakin@xxxxxxxx/

Regards,
á á á á á á