jackmanb added inline comments.
================
Comment at: llvm/lib/Target/BPF/BPFInstrInfo.td:684
+ let Inst{47-32} = addr{15-0}; // offset
+ let Inst{11-8} = val;
+ let Inst{7-4} = Opc.Value;
----------------
yonghong-song wrote:
> jackmanb wrote:
> > jackmanb wrote:
> > > jackmanb wrote:
> > > > Sorry I'm a beginner with the LLVM code, could you explain what `val`
> > > > does? I didn't notice this when I looked through here before.
> > > >
> > > > To try and get a clue I tried just removing this line and then
> > > > compiling the following code:
> > > >
> > > > ```C
> > > > // SPDX-License-Identifier: GPL-2.0
> > > > #include <stdatomic.h>
> > > >
> > > > #include <linux/bpf.h>
> > > > #include <bpf/bpf_helpers.h>
> > > > #include <bpf/bpf_tracing.h>
> > > >
> > > > __u64 test_data_64 = 0;
> > > > __u64 test1_result = 0;
> > > >
> > > > SEC("fentry/bpf_fentry_test1")
> > > > int BPF_PROG(test1, int a)
> > > > {
> > > > /* atomic_fetch_add(&test_data_64, 1); */
> > > > test1_result = __sync_fetch_and_add(&test_data_64, 1);
> > > > return 0;
> > > > }
> > > > ```
> > > >
> > > > And I was able to load and run the program, with the kernel on my WIP
> > > > branch: https://github.com/bjackman/linux-bpf/tree/wips/bpf-atomics-v0
> > > >
> > > > The result looks like this:
> > > >
> > > > ```shell
> > > > $ llvm-objdump -d atomics_test.o
> > > >
> > > > atomics_test.o: file format elf64-bpf
> > > >
> > > >
> > > > Disassembly of section fentry/bpf_fentry_test1:
> > > >
> > > > 0000000000000000 <test1>:
> > > > 0: b7 01 00 00 01 00 00 00 r1 = 1
> > > > 1: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0
> > > > ll
> > > > 3: db 12 00 00 01 00 00 00 r1 = atomic_fetch_add((u64
> > > > *)(r2 + 0), PLEASE submit a bug report to https://bugs.llvm.org/ and
> > > > include the crash backtrace.
> > > > Stack dump:
> > > > 0. Program arguments: llvm-objdump -d atomics_test.o
> > > > Segmentation fault
> > > > ```
> > > >
> > > > Aside from the fact that llvm-objdump crashed, the encoding `db 12 00
> > > > 00 01 00 00 00` seems correct to me. If I add the `let Inst{11-8} =
> > > > val` back in I get `db 12 00 00 01 01 00 00` which I don't understand.
> > > Ah wait, I guess this is adding a 3rd operand register? In my example
> > > it's unclear because it is R1 which is also the dst operand. I was
> > > envisaging we just have semantics like `src = atomic_fetch_add(dst+off,
> > > src)` but you are instead proposing `dst = atomic_fetch_add(dst+off,
> > > val)`?
> > Sorry I mean `dst = atomic_fetch_add(src+off, val)`
> > Sorry I'm a beginner with the LLVM code, could you explain what `val` does?
> > I didn't notice this when I looked through here before.
>
> For the below statement:
> test1_result = __sync_fetch_and_add(&test_data_64, 1);
>
> 'val' represents the register which holds the value '1'.
>
> bit 4-7 is also used in compare-and-exchange insn as you need a memory
> location, in-register old/new values.
>
> >
> > To try and get a clue I tried just removing this line and then compiling
> > the following code:
> >
> > ```C
> > // SPDX-License-Identifier: GPL-2.0
> > #include <stdatomic.h>
> >
> > #include <linux/bpf.h>
> > #include <bpf/bpf_helpers.h>
> > #include <bpf/bpf_tracing.h>
> >
> > __u64 test_data_64 = 0;
> > __u64 test1_result = 0;
> >
> > SEC("fentry/bpf_fentry_test1")
> > int BPF_PROG(test1, int a)
> > {
> > /* atomic_fetch_add(&test_data_64, 1); */
> > test1_result = __sync_fetch_and_add(&test_data_64, 1);
> > return 0;
> > }
> > ```
> >
> > And I was able to load and run the program, with the kernel on my WIP
> > branch: https://github.com/bjackman/linux-bpf/tree/wips/bpf-atomics-v0
> >
> > The result looks like this:
> >
> > ```shell
> > $ llvm-objdump -d atomics_test.o
> >
> > atomics_test.o: file format elf64-bpf
> >
> >
> > Disassembly of section fentry/bpf_fentry_test1:
> >
> > 0000000000000000 <test1>:
> > 0: b7 01 00 00 01 00 00 00 r1 = 1
> > 1: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
> > 3: db 12 00 00 01 00 00 00 r1 = atomic_fetch_add((u64 *)(r2 +
> > 0), PLEASE submit a bug report to https://bugs.llvm.org/ and include the
> > crash backtrace.
> > Stack dump:
> > 0. Program arguments: llvm-objdump -d atomics_test.o
> > Segmentation fault
> > ```
> >
>
> the crash may come from that the 'val' is not encoded properly. I will double
> check.
>
> > Aside from the fact that llvm-objdump crashed, the encoding `db 12 00 00 01
> > 00 00 00` seems correct to me. If I add the `let Inst{11-8} = val` back in
> > I get `db 12 00 00 01 01 00 00` which I don't understand.
>
> in this particular case, yes, as final asm code looks like
> r1 = atomic_fetch_add((u64 *)(r2 + 0), r1)
> where the value "r1" and the result "r1" shared the same register. A little
> bit compiler work is need to enforce "val" register and result register
> always the same.
>
> My current design allows:
> r3 = atomic_fetch_add((u64 *)(r2 + 0), r1)
> and I think this is more flexible as people may later on still use "r1". But
> let me know whether you prefer
> r1 = atomic_fetch_add((u64 *)(r2 + 0), r1)
> always.
>
>
Got it - this design will introduce some impedance mismatch with x86, since
XADD has only two operands (`r3 = atomic_fetch_add((u64 *)(r2 + 0), r1)` cannot
be implemented in a single instruction). It also hard-codes RAX as the third
operand for [cmp]xchg so my current kernel implementation hard-codes R0 -
that's also what Alexei supported in the separate email thread.
Will you be available to discuss this in the BPF office hours this week? There
are a few possible ways forward, we just need to decide which tradeoff is best.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D72184/new/
https://reviews.llvm.org/D72184
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