Re: MIPS eBPF JIT support on pre-32R2

[Alexander Lobakin <alobakin@dlink.ru>]

Alexander Lobakin писал 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@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

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

Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ