Re: [PATCH v7 net-next 1/3] filter: add Extended BPF interpreter and converter

[Alexei Starovoitov]

On Sun, Mar 9, 2014 at 7:45 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> On Sat, 2014-03-08 at 15:15 -0800, Alexei Starovoitov wrote:
>
>> +/**
>> + *   sk_run_filter_ext - run an extended filter
>> + *   @ctx: buffer to run the filter on
>> + *   @insn: filter to apply
>> + *
>> + * Decode and execute extended BPF instructions.
>> + * @ctx is the data we are operating on.
>> + * @filter is the array of filter instructions.
>> + */
>> +notrace u32 sk_run_filter_ext(void *ctx, const struct sock_filter_ext *insn)
>> +{
>> +     u64 stack[64];
>> +     u64 regs[16];
>> +     void *ptr;
>> +     u64 tmp;
>> +     int off;

First of all, great that you finally reviewed it! Feedback is appreciated :)

> Why is this 'notrace' ?

to avoid overhead of dummy call.
JITed filters are not adding this dummy call.
So 'notrace' on interpreter brings it to parity with JITed filters.

> 80 u64 on the stack, that is 640 bytes to run a filter ????

yes. that was described in commit log and in Doc...filter.txt:
"
- 16 4-byte stack slots for register spill-fill replaced with
  up to 512 bytes of multi-use stack space
"

For interpreter it is prohibitive to dynamically allocate stack space
that's why it just grabs 64*8 to run any program.
For JIT it's going to be close to zero for majority of filters, since
generated program will allocate only as much as was allowed
by sk_chk_filter_ext(). Only largest programs would need 'up to 512'.
This much stack would be needed for programs that need to use
large key/value pairs in their ebpf tables.
So far I haven't seen a program that approaches this limit,
but it seems to me that 512 is reasonable, since kernel warns on
functions with > 1k stack.

btw, current x86 jit just does 'subq  $96,%rsp',
I think ebpf jit should use the minimum amount of stack. Only amount
that is needed.
May be I'm over thinking it and having 'subq $512, %rsp' for JIT is also fine.
Let me know.

Thanks
Alexei
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