Date:   Thu, 25 Apr 2019 10:49:27 +0100
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, daniel@...earbox.net,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "oss-drivers@...ronome.com" <oss-drivers@...ronome.com>
Subject: Re: 32-bit zext time complexity (Was Re: [PATCH bpf-next]
 selftests/bpf: two scale tests)


> On 25 Apr 2019, at 08:25, Jiong Wang <jiong.wang@...ronome.com> wrote:
> 
> 
> Alexei Starovoitov writes:
> 
>> On Thu, Apr 25, 2019 at 12:07:06AM +0100, Jiong Wang wrote:
>>> 
>>> Alexei Starovoitov writes:
>>> 
>>>> Add two tests to check that sequence of 1024 jumps is verifiable.
>>>> 
>>>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>>> ---
>>>> tools/testing/selftests/bpf/test_verifier.c  | 70 ++++++++++++++++++++
>>>> tools/testing/selftests/bpf/verifier/scale.c | 18 +++++
>>> 
>>> I am rebasing 32-bit opt pass on top of latest bpf-next and found these new
>>> tests take more than 20 minutes to run and had not finished after that.
>>> 
>>> The reason the following insn filling insde bpf_fill_scale1 is generating
>>> nearly 1M insn whose results are recognized as safe to be poisoned.
>>> 
>>>  bpf_fill_scale1:
>>>    while (i < MAX_TEST_INSNS - 1025)
>>>      insn[i++] = BPF_ALU64_IMM(BPF_MOV, BPF_REG_0, 42);
>>> 
>>> For each hi32 poisoning, there will be one call to "bpf_patch_insn_data"
>>> which actually is not cheap (adjust jump insns, insn aux info etc). Now,
>>> 1M call to it has exhausted server resources as described, 20minutes running
>>> still not finished.
>>> 
>>> For real world applications, we don't do hi32 poisoning, and there isn't much
>>> lo32 zext. Benchmarking those bpf programs inside Cilium shows the final
>>> zext pass adds about 8% ~ 15% verification time.
>>> 
>>> The zext pass based on top of "bpf_patch_insn_data" looks more and more is
>>> not the best approach to utilize the read32 analysis results.
>>> 
>>> Previously, in v1 cover letter, I listed some of my other thoughts on how to
>>> utilize the liveness analysis results:
>>> 
>>>   1 Minor change on back-end JIT hook, also pass aux_insn information to
>>>     back-ends so they could have per insn information and they could do
>>>     zero extension for the marked insn themselves using the most
>>>     efficient native insn.
>>> 
>>>   2 Introduce zero extension insn for eBPF. Then verifier could insert
>>>     the new zext insn instead of lshift + rshift. zext could be JITed
>>>     more efficiently.
>>> 
>>>   3 Otherwise JIT back-ends need to do peephole to catch lshift + rshift
>>>     and turn them into native zext.
>> 
>> all options sounds like hacks to workaround inefficient bpf_patch_insn_data().
>> Especially option 2 will work only because single insn is replaced
>> with another insn ?
> 
> Option 1 should be a generic solution. It is passing verifier analysis
> results generated by insn walk down to JIT back-ends. The information
> passed down could be any analysis result useful for JIT code-gen.
> 
>> Let's fix the algo of bpf_patch_insn_data() instead, so that 1 insn -> 2+ insn
>> is also fast.
> 
> The issue with 1 insn -> 2+ insn should be calling of bpf_adj_branches
> which is doing another for_each_insn_in_prog traversal, so the zext
> insertion becomes something like:
> 
>  for_each_insn_in_prog
>  ...
>     if (zext)
>     ...
>       for_each_insn_in_prog
> 
> which is quadratic. One solution

s/solution/mitigation/

> is we chain all branch insns during
> previous insn traversal in for example cfg check, and keep the information
> somewhere info bpf_prog (env->insn_aux_data is a good place to keep such
> information, but insn patch helpers are supposed to work with bpf_prog)
> then bpf_adj_branches could traversal this chain instead of iterating
> through all insns.
> 
> Regards,
> Jiong

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