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From: Yonghong Song <yhs@fb.com>
To: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Tony Ambardar <Tony.Ambardar@gmail.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH 08/14] bpf/tests: Add tests for ALU operations implemented with function calls
Date: Thu, 29 Jul 2021 15:54:27 -0700	[thread overview]
Message-ID: <a492ce4f-4fc6-b6d6-74a5-9ded35d84608@fb.com> (raw)
In-Reply-To: <CAM1=_QQRuH2K3fMDJCYJuDtTmziqcmtcr31hQeQe-kCkXVC4gA@mail.gmail.com>



On 7/29/21 2:17 PM, Johan Almbladh wrote:
> On Thu, Jul 29, 2021 at 1:52 AM Yonghong Song <yhs@fb.com> wrote:
>>> +             /*
>>> +              * Register (non-)clobbering test, in the case where a 32-bit
>>> +              * JIT implements complex ALU64 operations via function calls.
>>> +              */
>>> +             "INT: Register clobbering, R1 updated",
>>> +             .u.insns_int = {
>>> +                     BPF_ALU32_IMM(BPF_MOV, R0, 0),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R1, 123456789),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R2, 2),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R3, 3),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R4, 4),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R5, 5),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R6, 6),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R7, 7),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R8, 8),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R9, 9),
>>> +                     BPF_ALU64_IMM(BPF_DIV, R1, 123456789),
>>> +                     BPF_JMP_IMM(BPF_JNE, R0, 0, 10),
>>> +                     BPF_JMP_IMM(BPF_JNE, R1, 1, 9),
>>> +                     BPF_JMP_IMM(BPF_JNE, R2, 2, 8),
>>> +                     BPF_JMP_IMM(BPF_JNE, R3, 3, 7),
>>> +                     BPF_JMP_IMM(BPF_JNE, R4, 4, 6),
>>> +                     BPF_JMP_IMM(BPF_JNE, R5, 5, 5),
>>> +                     BPF_JMP_IMM(BPF_JNE, R6, 6, 4),
>>> +                     BPF_JMP_IMM(BPF_JNE, R7, 7, 3),
>>> +                     BPF_JMP_IMM(BPF_JNE, R8, 8, 2),
>>> +                     BPF_JMP_IMM(BPF_JNE, R9, 9, 1),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R0, 1),
>>> +                     BPF_EXIT_INSN(),
>>> +             },
>>> +             INTERNAL,
>>> +             { },
>>> +             { { 0, 1 } }
>>> +     },
>>> +     {
>>> +             "INT: Register clobbering, R2 updated",
>>> +             .u.insns_int = {
>>> +                     BPF_ALU32_IMM(BPF_MOV, R0, 0),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R1, 1),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R2, 2 * 123456789),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R3, 3),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R4, 4),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R5, 5),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R6, 6),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R7, 7),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R8, 8),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R9, 9),
>>> +                     BPF_ALU64_IMM(BPF_DIV, R2, 123456789),
>>> +                     BPF_JMP_IMM(BPF_JNE, R0, 0, 10),
>>> +                     BPF_JMP_IMM(BPF_JNE, R1, 1, 9),
>>> +                     BPF_JMP_IMM(BPF_JNE, R2, 2, 8),
>>> +                     BPF_JMP_IMM(BPF_JNE, R3, 3, 7),
>>> +                     BPF_JMP_IMM(BPF_JNE, R4, 4, 6),
>>> +                     BPF_JMP_IMM(BPF_JNE, R5, 5, 5),
>>> +                     BPF_JMP_IMM(BPF_JNE, R6, 6, 4),
>>> +                     BPF_JMP_IMM(BPF_JNE, R7, 7, 3),
>>> +                     BPF_JMP_IMM(BPF_JNE, R8, 8, 2),
>>> +                     BPF_JMP_IMM(BPF_JNE, R9, 9, 1),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R0, 1),
>>> +                     BPF_EXIT_INSN(),
>>> +             },
>>> +             INTERNAL,
>>> +             { },
>>> +             { { 0, 1 } }
>>> +     },
>>
>> It looks like the above two tests, "R1 updated" and "R2 updated" should
>> be very similar and the only difference is one immediate is 123456789
>> and another is 2 * 123456789. But for generated code, they all just have
>> the final immediate. Could you explain what the difference in terms of
>> jit for the above two tests?
> 
> When a BPF_CALL instruction is executed, the eBPF assembler have
> already saved any caller-saved registers that must be preserved, put
> the arguments in R1-R5, and expects a return value in R0. It is just
> for the JIT to emit the call.
> 
> Not so when an eBPF instruction is implemented by a function call,
> like ALU64 DIV in a 32-bit JIT. In this case, the function call is
> unexpected by the eBPF assembler, and must be invisible to it. Now the
> JIT must take care of saving all caller-saved registers on stack, put
> the operands in the right argument registers, put the return value in
> the destination register, and finally restore all caller-saved
> registers without overwriting the computed result.
> 
> The test checks that all other registers retain their value after such
> a hidden function call. However, one register will contain the result.
> In order to verify that all registers are saved and restored properly,
> we must vary the destination and run it two times. It is not the
> result of the operation that its tested, it is absence of possible
> side effects.
> 
> I can put a more elaborate description in the comment to explain this.

Indeed, an elaborate description as comments will be great.

> 
>>
>>> +     {
>>> +             /*
>>> +              * Test 32-bit JITs that implement complex ALU64 operations as
>>> +              * function calls R0 = f(R1, R2), and must re-arrange operands.
>>> +              */
[...]

  reply	other threads:[~2021-07-29 22:54 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-28 17:04 [PATCH 00/14] bpf/tests: Extend the eBPF test suite Johan Almbladh
2021-07-28 17:04 ` [PATCH 01/14] bpf/tests: Add BPF_JMP32 test cases Johan Almbladh
2021-07-28 22:31   ` Yonghong Song
2021-07-29 21:30     ` Johan Almbladh
2021-07-28 17:04 ` [PATCH 02/14] bpf/tests: Add BPF_MOV tests for zero and sign extension Johan Almbladh
2021-07-28 22:36   ` Yonghong Song
2021-07-28 17:04 ` [PATCH 03/14] bpf/tests: Fix typos in test case descriptions Johan Almbladh
2021-07-28 22:43   ` Yonghong Song
2021-07-28 17:04 ` [PATCH 04/14] bpf/tests: Add more tests of ALU32 and ALU64 bitwise operations Johan Almbladh
2021-07-28 22:53   ` Yonghong Song
2021-07-28 17:04 ` [PATCH 05/14] bpf/tests: Add more ALU32 tests for BPF_LSH/RSH/ARSH Johan Almbladh
2021-07-28 22:57   ` Yonghong Song
2021-07-28 17:04 ` [PATCH 06/14] bpf/tests: Add more BPF_LSH/RSH/ARSH tests for ALU64 Johan Almbladh
2021-07-28 23:30   ` Yonghong Song
2021-07-29 12:34     ` Johan Almbladh
2021-07-29 15:39       ` Yonghong Song
2021-07-28 17:04 ` [PATCH 07/14] bpf/tests: Add more ALU64 BPF_MUL tests Johan Almbladh
2021-07-28 23:32   ` Yonghong Song
2021-07-29 21:21     ` Johan Almbladh
2021-07-28 17:04 ` [PATCH 08/14] bpf/tests: Add tests for ALU operations implemented with function calls Johan Almbladh
2021-07-28 23:52   ` Yonghong Song
2021-07-29 21:17     ` Johan Almbladh
2021-07-29 22:54       ` Yonghong Song [this message]
2021-07-28 17:04 ` [PATCH 09/14] bpf/tests: Add word-order tests for load/store of double words Johan Almbladh
2021-07-28 23:54   ` Yonghong Song
2021-07-28 17:04 ` [PATCH 10/14] bpf/tests: Add branch conversion JIT test Johan Almbladh
2021-07-28 23:58   ` Yonghong Song
2021-07-29 12:45     ` Johan Almbladh
2021-07-29 15:46       ` Yonghong Song
2021-07-29  0:55   ` Yonghong Song
2021-07-29 13:24     ` Johan Almbladh
2021-07-29 15:50       ` Yonghong Song
2021-07-28 17:04 ` [PATCH 11/14] bpf/tests: Add test for 32-bit context pointer argument passing Johan Almbladh
2021-07-29  0:09   ` Yonghong Song
2021-07-29 13:29     ` Johan Almbladh
2021-07-29 15:50       ` Yonghong Song
2021-07-28 17:05 ` [PATCH 12/14] bpf/tests: Add tests for atomic operations Johan Almbladh
2021-07-29  0:36   ` Yonghong Song
2021-07-28 17:05 ` [PATCH 13/14] bpf/tests: Add tests for BPF_CMPXCHG Johan Almbladh
2021-07-29  0:45   ` Yonghong Song
2021-07-28 17:05 ` [PATCH 14/14] bpf/tests: Add tail call test suite Johan Almbladh
2021-07-29  2:56   ` Yonghong Song
2021-07-29 20:44     ` Johan Almbladh

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