Messages in this thread | | | Date | Fri, 06 Dec 2019 10:20:40 +0300 | From | Alexander Lobakin <> | Subject | Re: MIPS eBPF JIT support on pre-32R2 |
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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@kernel.org/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, ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ
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