Re: [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

[Wangnan (F)]

On 2015/8/5 10:05, Wangnan (F) wrote:
> Send again since llvmdev is moved to llvm-dev@xxxxxxxxxxxxxx
>
> On 2015/8/5 9:58, Wangnan (F) wrote:
> > > On 2015/8/4 3:44, Alexei Starovoitov wrote:
> > > > On 7/31/15 3:18 AM, Wangnan (F) wrote:
> > >
> > > [SNIP]
> > >
> > > > didn't have time to look at it.
> > > > from your llvm patches looks like you've got quite experienced
> > > > with it already :)
> > > >
> > > > > I'll post 2 LLVM patches by replying this mail. Please have a look 
> > > > > and
> > > > > help me
> > > > > send them to LLVM if you think my code is correct.
> > > >
> > > > patch 1:
> > > > I don't quite understand the purpose of builtin_dwarf_cfa
> > > > returning R11. It's a special register seen inside llvm codegen
> > > > only. It doesn't have kernel meaning.
> > >
> > > Kernel side verifier allows us to do arithmetic computation using 
> > > two local variable
> > > address or local variable address and R11. Therefore, we can compute 
> > > the location
> > > of a local variable using:
> > >
> > >   mark = &my_var_a - __builtin_frame_address(0);
> > >
> > > If the stack allocation is fixed (if the location is never reused), 
> > > the above 'mark'
> > > can be uniquely identify a local variable. That's why I'm 
> > > interesting in it. However
> > > I'm not sure whether the prerequestion is hold.
> > >
> > > > patch 2:
> > > > do we really need to hack clang?
> > > > Can you just define a function that aliases to intrinsic,
> > > > like we do for ld_abs/ld_ind ?
> > > > void bpf_store_half(void *skb, u64 off, u64 val) 
> > > > asm("llvm.bpf.store.half");
> > > > then no extra patches necessary.

And for this:

I tried this test function:

void bpf_store_half(void *skb, int off, int val) asm("llvm.bpf.store.half");
int func()
{
        bpf_store_half(0, 0, 0);
        return 0;
}

Compiled with:

$ clang -g -target bpf -O2 -S -c test.c

And get this:

        .text
        .globl  func
        .align  8
func:                                   # @func
# BB#0:                                 # %entry
        mov     r1, 0
        mov     r2, 0
        mov     r3, 0
        call    llvm.bpf.store.half
        mov     r0, 0
        ret

Without -S, it generate a function relocation:

$ objdump -r ./test.o

./test.o:     file format elf64-little

RELOCATION RECORDS FOR [.text]:
OFFSET           TYPE              VALUE
0000000000000018 UNKNOWN           llvm.bpf.store.half


It doesn't work as you suggestion. I think we still need to do something
in clang frontend, or it can only be used in '.ll'.

Thank you.


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