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From: Johan Almbladh <johan.almbladh@anyfinetworks.com>
To: Yonghong Song <yhs@fb.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 06/14] bpf/tests: Add more BPF_LSH/RSH/ARSH tests for ALU64
Date: Thu, 29 Jul 2021 14:34:02 +0200	[thread overview]
Message-ID: <CAM1=_QQJ+uYXuU_nOVb3djW-G8wJs4Azz36pXk8mO3vQBuVouQ@mail.gmail.com> (raw)
In-Reply-To: <b134e3bc-a9f7-6c4f-21fe-8d5068ac029e@fb.com>

On Thu, Jul 29, 2021 at 1:30 AM Yonghong Song <yhs@fb.com> wrote:
> > @@ -4139,6 +4139,106 @@ static struct bpf_test tests[] = {
> >               { },
> >               { { 0, 0x80000000 } },
> >       },
> > +     {
> > +             "ALU64_LSH_X: Shift < 32, low word",
> > +             .u.insns_int = {
> > +                     BPF_LD_IMM64(R0, 0x0123456789abcdefLL),
> > +                     BPF_ALU32_IMM(BPF_MOV, R1, 12),
> > +                     BPF_ALU64_REG(BPF_LSH, R0, R1),
> > +                     BPF_EXIT_INSN(),
> > +             },
> > +             INTERNAL,
> > +             { },
> > +             { { 0, 0xbcdef000 } }
>
> In bpf_test struct, the result is defined as __u32
>          struct {
>                  int data_size;
>                  __u32 result;
>          } test[MAX_SUBTESTS];
>
> But the above result 0xbcdef000 does not really capture the bpf program
> return value, which should be 0x3456789abcdef000.
> Can we change "result" type to __u64 so the result truly captures the
> program return value?

This was also my though at first, but I don't think that is possible.
As I understand it, the eBPF functions have the prototype int
func(struct *ctx). While the context pointer will have a different
size on 32-bit and 64-bit architectures, the return value will always
be 32 bits on most, or all, platforms.

> We have several other similar cases for the rest of this patch.

I have used two ways to check the full 64-bit result in such cases.

1) Load the expected result as a 64-bit value in a register. Then jump
conditionally if the result matches this value or not. The jump
destinations each set a distinct value in R0, which is finally
examined as the result.

2) Run the test twice. The first one returns the low 32-bits of R0.
The second adds a 32-bit right shift to return the high 32 bits.

When I first wrote the tests I tried to use as few complex
instructions not under test as possible, in order to test each
instruction in isolation. Since the 32-bit right shift is a much
simpler operation than conditional jumps, at least in the 32-bit MIPS
JIT, I chose method (2) for most of the tests. Existing tests seem to
use method (1), so in some cases I used that instead when adding more
tests of the same operation. The motivation for the simple one-by-one
tests is mainly convenience and better diagnostics during JIT
development. Both methods (1) and (2) are equally valid of course.

By the way, thanks a lot for the review, Yonghong!

Johan

  reply	other threads:[~2021-07-29 12:34 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 [this message]
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
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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