netdev - Re: [PATCH v2 bpf-next 3/4] selftests/bpf: Add profiler test

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Date:   Thu, 8 Oct 2020 23:49:27 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <john.fastabend@...il.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 3/4] selftests/bpf: Add profiler test



On 10/8/20 6:12 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> The main purpose of the profiler test to check different llvm generation
> patterns to make sure the verifier can load these large programs.
> 
> Note that profiler.inc.h test doesn't follow strict kernel coding style.
> The code was formatted in the kernel style, but variable declarations are
> kept as-is to preserve original llvm IR pattern.
> 
> profiler1.c should pass with older and newer llvm
> 
> profiler[23].c may fail on older llvm that don't have:
> https://reviews.llvm.org/D85570

Not sure but the below equivalent URL may be more intuitive:
   https://reviews.llvm.org/D85570

> because llvm may do speculative code motion optimization that
> will generate code like this:
> 
> // r9 is a pointer to map_value
> // r7 is a scalar
> 17:       bf 96 00 00 00 00 00 00 r6 = r9
> 18:       0f 76 00 00 00 00 00 00 r6 += r7
> 19:       a5 07 01 00 01 01 00 00 if r7 < 257 goto +1
> 20:       bf 96 00 00 00 00 00 00 r6 = r9
> // r6 is used here
> 
> The verifier will reject such code with the error:
> "math between map_value pointer and register with unbounded min value is not allowed"
> At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and
> the insn 20 undoes map_value addition. It is currently impossible for the
> verifier to understand such speculative pointer arithmetic. Hence llvm D85570
> addresses it on the compiler side.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

Acked-by: Yonghong Song <yhs@...com>