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From: Yonghong Song <yhs@fb.com>
To: John Fastabend <john.fastabend@gmail.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: fix a verifier failure with xor
Date: Tue, 1 Sep 2020 22:43:03 -0700	[thread overview]
Message-ID: <c26f372b-748e-2141-33bc-00d40c5e4205@fb.com> (raw)
In-Reply-To: <5f4f2d24485ac_5f398208c@john-XPS-13-9370.notmuch>



On 9/1/20 10:27 PM, John Fastabend wrote:
> Yonghong Song wrote:
>>
>>
>> On 9/1/20 1:07 PM, Andrii Nakryiko wrote:
>>> On Mon, Aug 24, 2020 at 11:47 PM Yonghong Song <yhs@fb.com> wrote:
>>>>
>>>> bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12.
>>>> Compared to llvm 10, llvm 11 and 12 generates xor instruction which
>>>
>>> Does this mean that some perfectly working BPF programs will now fail
>>> to verify on older kernels, if compiled with llvm 11 or llvm 12? If
>>
>> Right.
>>
>>> yes, is there something that one can do to prevent Clang from using
>>> xor in such situations?
>>
>> The xor is generated by the combination of llvm simplifyCFG and
>> instrCombine phase.
> 
> Another option would be to move it out of the isAsCheapAsAMove on the

John, do you mean the following change?

diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td 
b/llvm/lib/Target/BPF/BPFInstrInfo.td
index 4298e2eaec04..7448a2499d40 100644
--- a/llvm/lib/Target/BPF/BPFInstrInfo.td
+++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
@@ -293,9 +293,9 @@ let isAsCheapAsAMove = 1 in {
    defm AND : ALU<BPF_AND, "&=", and>;
    defm SLL : ALU<BPF_LSH, "<<=", shl>;
    defm SRL : ALU<BPF_RSH, ">>=", srl>;
-  defm XOR : ALU<BPF_XOR, "^=", xor>;
    defm SRA : ALU<BPF_ARSH, "s>>=", sra>;
  }
+  defm XOR : ALU<BPF_XOR, "^=", xor>;
    defm MUL : ALU<BPF_MUL, "*=", mul>;
    defm DIV : ALU<BPF_DIV, "/=", udiv>;
  }

Tried the above change with latest trunk. xor still generated :-(
I did not trace down to exact llvm optimization location for this
particular optimization instance.

> llvm side. But, probably better to force the workaround until kernels
> get support. Even with it being more expensive it wouldn't mean we never
> get it so likely not a great idea. Just thought it might be worth
> mentioning. If you have your own llvm and don't have these kernels yet
> it looks like a win.
> 
>>
>> The following is a hack to prevent compiler from generating xor's.

  reply	other threads:[~2020-09-02  5:43 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-25  6:46 [PATCH bpf-next 0/2] fix a verifier failure with xor Yonghong Song
2020-08-25  6:46 ` [PATCH bpf-next 1/2] bpf: " Yonghong Song
2020-08-26  1:58   ` Alexei Starovoitov
2020-08-26  3:36     ` Yonghong Song
2020-08-26 22:06       ` John Fastabend
2020-08-27  5:12         ` Alexei Starovoitov
2020-08-27 18:43           ` John Fastabend
2020-09-01 20:07   ` Andrii Nakryiko
2020-09-02  2:17     ` Yonghong Song
2020-09-02  5:27       ` John Fastabend
2020-09-02  5:43         ` Yonghong Song [this message]
2020-09-04  5:29           ` John Fastabend
2020-09-02  9:33       ` Toke Høiland-Jørgensen
2020-09-02 14:21         ` Alexei Starovoitov
2020-09-02 15:01           ` Toke Høiland-Jørgensen
2020-09-02 21:40             ` Alexei Starovoitov
2020-08-25  6:46 ` [PATCH bpf-next 2/2] selftests/bpf: add verifier tests for xor operation Yonghong Song

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