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: > > 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! 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 :) Thanks, Paul
