Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1205558 > unrolled thread

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

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function  for outputing data to perf event "Wangnan (F)" <wangnan0@huawei.com> - 2015-08-12 04:40 +0200
    Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf:  Introduce function for outputing data to perf event Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2015-08-12 07:00 +0200
      Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce  function for outputing data to perf event "Wangnan (F)" <wangnan0@huawei.com> - 2015-08-12 07:50 +0200
        Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf:  Introduce function for outputing data to perf event Brenden Blanco <bblanco@gmail.com> - 2015-08-12 15:20 +0200
          Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce  function for outputing data to perf event "Wangnan (F)" <wangnan0@huawei.com> - 2015-08-13 08:30 +0200

#1205558 — Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

On 2015/8/4 3:44, Alexei Starovoitov wrote:

[SNIP]
>> 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.
>
>
[SNIP]
> 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.

Hi Alexei,

By two weeks researching, I have to give you a sad answer that:

target specific intrinsic is not work.

I tried target specific intrinsic. However, LLVM isolates backend and
frontend, and there's no way to pass language level type information
to backend code.

Think about a program like this:

struct strA { int a; }
struct strB { int b; }
int func() {
   struct strA a;
   struct strB b;

   a.a = 1;
   b.b = 2;
   bpf_output(gettype(a), &a);
   bpf_output(gettype(b), &b);
   return 0;
}

BPF backend can't (and needn't) tell the difference between local
variables a and b in theory. In LLVM implementation, it filters type
information out using ComputeValueVTs().  Please have a look at
SelectionDAGBuilder::visitIntrinsicCall in
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp and
SelectionDAGBuilder::visitTargetIntrinsic in the same file. in
visitTargetIntrinsic, ComputeValueVTs acts as a barrier which strips
type information out from CallInst ("I"), and leave SDValue and SDVTList
("Ops" and "VTs") to target code. SDValue and SDVTList are wrappers of
EVT and MVT, all information we concern won't be passed here.

I think now we have 2 choices:

1. Hacking into clang, implement target specific builtin function. Now I
    have worked out a ugly but workable patch which setup a builtin 
function:
    __builtin_bpf_typeid(), which accepts local or global variable then
    returns different constant for different types.

2. Implementing an LLVM intrinsic call (llvm.typeid), make it be 
processed in
    visitIntrinsicCall(). I think we can get something useful if it is 
processed
    with that function.

The next thing should be generating debug information to map type and
constants which issued by __builtin_bpf_typeid() or llvm.typeid. Now we
have a crazy idea that, if we limit the name of the structure to 8 bytes,
we can insert the name into a u64, then there would be no need to consider
type information in DWARF. For example, in the above sample code, gettype(a)
will issue 0x0000000041727473 because its type is "strA". What do you think?

Thank you.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [next] | [standalone]

#1205584 — Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

On Wed, Aug 12, 2015 at 10:34:43AM +0800, Wangnan (F) via llvm-dev wrote:
> 
> Think about a program like this:
> 
> struct strA { int a; }
> struct strB { int b; }
> int func() {
>   struct strA a;
>   struct strB b;
> 
>   a.a = 1;
>   b.b = 2;
>   bpf_output(gettype(a), &a);
>   bpf_output(gettype(b), &b);
>   return 0;
> }
> 
> BPF backend can't (and needn't) tell the difference between local
> variables a and b in theory. In LLVM implementation, it filters type
> information out using ComputeValueVTs().  Please have a look at
> SelectionDAGBuilder::visitIntrinsicCall in
> lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp and
> SelectionDAGBuilder::visitTargetIntrinsic in the same file. in
> visitTargetIntrinsic, ComputeValueVTs acts as a barrier which strips
> type information out from CallInst ("I"), and leave SDValue and SDVTList
> ("Ops" and "VTs") to target code. SDValue and SDVTList are wrappers of
> EVT and MVT, all information we concern won't be passed here.
> 
> I think now we have 2 choices:
> 
> 1. Hacking into clang, implement target specific builtin function. Now I
>    have worked out a ugly but workable patch which setup a builtin function:
>    __builtin_bpf_typeid(), which accepts local or global variable then
>    returns different constant for different types.
> 
> 2. Implementing an LLVM intrinsic call (llvm.typeid), make it be processed
> in
>    visitIntrinsicCall(). I think we can get something useful if it is
> processed
>    with that function.

Yeah. You're right about pure target intrinsics.
I think llvm.typeid might work. imo it's cleaner than
doing it at clang level.

> The next thing should be generating debug information to map type and
> constants which issued by __builtin_bpf_typeid() or llvm.typeid. Now we
> have a crazy idea that, if we limit the name of the structure to 8 bytes,
> we can insert the name into a u64, then there would be no need to consider
> type information in DWARF. For example, in the above sample code, gettype(a)
> will issue 0x0000000041727473 because its type is "strA". What do you think?

that's way too hacky.
I was thinking when compiling we can keep llvm ir along with .o
instead of dwarf and extract type info from there.
dwarf has names and other things that we don't need. We only
care about actual field layout of the structs.
But it probably won't be easy to parse llvm ir on perf side
instead of dwarf.

btw, if you haven't looked at iovisor/bcc, there we're solving
similar problem differently. There we use clang rewriter, so all
structs fields are visible at this level, then we use bpf backend
in JIT mode and push bpf instructions into the kernel on the fly
completely skipping ELF and .o
For example in:
https://github.com/iovisor/bcc/blob/master/examples/distributed_bridge/tunnel.c
when you see
struct ethernet_t {
  unsigned long long  dst:48;
  unsigned long long  src:48;
  unsigned int        type:16;
} BPF_PACKET_HEADER;
struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
... ethernet->src ...
is recognized by clang rewriter and ->src is converted to a different
C code that is sent again into clang.
So there is no need to use dwarf or patch clang/llvm. clang rewriter
has all the info.
I'm not sure you can live with clang/llvm on the host where you
want to run the tracing bits, but if you can that's an easier option.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1205597 — Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

On 2015/8/12 12:57, Alexei Starovoitov wrote:
> On Wed, Aug 12, 2015 at 10:34:43AM +0800, Wangnan (F) via llvm-dev wrote:
>> Think about a program like this:
>>
>> struct strA { int a; }
>> struct strB { int b; }
>> int func() {
>>    struct strA a;
>>    struct strB b;
>>
>>    a.a = 1;
>>    b.b = 2;
>>    bpf_output(gettype(a), &a);
>>    bpf_output(gettype(b), &b);
>>    return 0;
>> }
>>
>> BPF backend can't (and needn't) tell the difference between local
>> variables a and b in theory. In LLVM implementation, it filters type
>> information out using ComputeValueVTs().  Please have a look at
>> SelectionDAGBuilder::visitIntrinsicCall in
>> lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp and
>> SelectionDAGBuilder::visitTargetIntrinsic in the same file. in
>> visitTargetIntrinsic, ComputeValueVTs acts as a barrier which strips
>> type information out from CallInst ("I"), and leave SDValue and SDVTList
>> ("Ops" and "VTs") to target code. SDValue and SDVTList are wrappers of
>> EVT and MVT, all information we concern won't be passed here.
>>
>> I think now we have 2 choices:
>>
>> 1. Hacking into clang, implement target specific builtin function. Now I
>>     have worked out a ugly but workable patch which setup a builtin function:
>>     __builtin_bpf_typeid(), which accepts local or global variable then
>>     returns different constant for different types.
>>
>> 2. Implementing an LLVM intrinsic call (llvm.typeid), make it be processed
>> in
>>     visitIntrinsicCall(). I think we can get something useful if it is
>> processed
>>     with that function.
> Yeah. You're right about pure target intrinsics.
> I think llvm.typeid might work. imo it's cleaner than
> doing it at clang level.
>
>> The next thing should be generating debug information to map type and
>> constants which issued by __builtin_bpf_typeid() or llvm.typeid. Now we
>> have a crazy idea that, if we limit the name of the structure to 8 bytes,
>> we can insert the name into a u64, then there would be no need to consider
>> type information in DWARF. For example, in the above sample code, gettype(a)
>> will issue 0x0000000041727473 because its type is "strA". What do you think?
> that's way too hacky.
> I was thinking when compiling we can keep llvm ir along with .o
> instead of dwarf and extract type info from there.
> dwarf has names and other things that we don't need. We only
> care about actual field layout of the structs.
> But it probably won't be easy to parse llvm ir on perf side
> instead of dwarf.

Shipping both llvm IR and .o to perf makes it harder to use. I'm
not sure whether it is a good idea. If we are unable to encode the
structure using a u64, let's still dig into dwarf.

We have another idea that we can utilize dwarf's existing feature.
For example, when __buildin_bpf_typeid() get called, define an enumerate
type in dwarf info, so you'll find:

  <1><2a>: Abbrev Number: 2 (DW_TAG_enumeration_type)
     <2b>   DW_AT_name        : (indirect string, offset: 0xec): TYPEINFO
     <2f>   DW_AT_byte_size   : 4
     <30>   DW_AT_decl_file   : 1
     <31>   DW_AT_decl_line   : 3
  <2><32>: Abbrev Number: 3 (DW_TAG_enumerator)
     <33>   DW_AT_name        : (indirect string, offset: 0xcc): 
__typeinfo_strA
     <37>   DW_AT_const_value : 2
  <2><38>: Abbrev Number: 3 (DW_TAG_enumerator)
     <39>   DW_AT_name        : (indirect string, offset: 0xdc): 
__typeinfo_strB
     <3d>   DW_AT_const_value : 3

or this:

  <3><54>: Abbrev Number: 4 (DW_TAG_variable)
     <55>   DW_AT_const_value : 2
     <66>   DW_AT_name        : (indirect string, offset: 0x1e): 
__typeinfo_strA
     <6a>   DW_AT_decl_file   : 1
     <6b>   DW_AT_decl_line   : 29
     <6c>   DW_AT_type        : <0x72>

then from DW_AT_name and DW_AT_const_value we can do the mapping. 
Drawback is that
all __typeinfo_ prefixed names become reserved.


> btw, if you haven't looked at iovisor/bcc, there we're solving
> similar problem differently. There we use clang rewriter, so all
> structs fields are visible at this level, then we use bpf backend
> in JIT mode and push bpf instructions into the kernel on the fly
> completely skipping ELF and .o
> For example in:
> https://github.com/iovisor/bcc/blob/master/examples/distributed_bridge/tunnel.c
> when you see
> struct ethernet_t {
>    unsigned long long  dst:48;
>    unsigned long long  src:48;
>    unsigned int        type:16;
> } BPF_PACKET_HEADER;
> struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
> ... ethernet->src ...
> is recognized by clang rewriter and ->src is converted to a different
> C code that is sent again into clang.
> So there is no need to use dwarf or patch clang/llvm. clang rewriter
> has all the info.

Could you please give us further information about your clang rewriter?
I guess you need a new .so when injecting those code into kernel?

> I'm not sure you can live with clang/llvm on the host where you
> want to run the tracing bits, but if you can that's an easier option.
>

I'm not sure. Our target platform should be embedded devices like 
smartphone.
Bringing full clang/llvm environment there is not acceptable.

Thank you.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1206075 — Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

Hi Wangnan, I've been authoring the BCC development, so I'll answer
those specific questions.
>
>
> Could you please give us further information about your clang rewriter?
> I guess you need a new .so when injecting those code into kernel?

The rewriter runs all of its passes in a single process, creating no
files on disk and having no external dependencies in terms of
toolchain.
1. Entry point: bpf_module_create() - C API call to create module, can
take filename or directly a c string with the full contents of the
program
2. Convert contents into a clang memory buffer
3. Set up a clang driver::CompilerInvocation in the style of the clang
interpreter example
4. Run a rewriter pass over the memory buffer file, annotating and/or
doing BPF specific magic on the input source
 a. Open BPF maps with a call to bpf_create_map directly
 b. Convert references to map operations with the specific FD of the new map
 c. Convert arguments to bpf_probe_read calls as needed
 d. Collect the externed function names to avoid section() hack in the language
5. Re-run the CompilerInvocation on the modified sources
6. JIT the llvm::Module to bpf arch
7. Load the resulting in-memory ".o" to bpf_prog_load, keeping the FD
alive in the compiler process
8. Attach the FD as necessary to perf events, socket, tc, etc.
9. goto 1

The above steps are captured in the BCC github repo in src/cc, with
the clang specific bits inside of the frontends/clang subdirectory.

> I'm not sure. Our target platform should be embedded devices like
> smartphone.
> Bringing full clang/llvm environment there is not acceptable.

The artifact from the build process of BCC is a shared library, which
has the clang/llvm .a embedded within them. It is not yet a single
binary, but not unfeasible to make it so. The clang toolchain itself
does not need to exist on the target. I have not attempted to
cross-compile BCC to any architecture, currently x86_64 only.

If you have more BCC specific questions not involving clang/llvm,
perhaps you can ping Alexei/myself off of the llvm-dev list, in case
this discussion is not relevant to them.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1206533 — Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

Thank you for your reply.

Add He Kuang to CC list.

On 2015/8/12 21:15, Brenden Blanco wrote:
> Hi Wangnan, I've been authoring the BCC development, so I'll answer
> those specific questions.
>>
>> Could you please give us further information about your clang rewriter?
>> I guess you need a new .so when injecting those code into kernel?
> The rewriter runs all of its passes in a single process, creating no
> files on disk and having no external dependencies in terms of
> toolchain.
> 1. Entry point: bpf_module_create() - C API call to create module, can
> take filename or directly a c string with the full contents of the
> program
> 2. Convert contents into a clang memory buffer
> 3. Set up a clang driver::CompilerInvocation in the style of the clang
> interpreter example
> 4. Run a rewriter pass over the memory buffer file, annotating and/or
> doing BPF specific magic on the input source
>   a. Open BPF maps with a call to bpf_create_map directly
>   b. Convert references to map operations with the specific FD of the new map
>   c. Convert arguments to bpf_probe_read calls as needed
>   d. Collect the externed function names to avoid section() hack in the language
> 5. Re-run the CompilerInvocation on the modified sources
> 6. JIT the llvm::Module to bpf arch
> 7. Load the resulting in-memory ".o" to bpf_prog_load, keeping the FD
> alive in the compiler process
> 8. Attach the FD as necessary to perf events, socket, tc, etc.
> 9. goto 1
>
> The above steps are captured in the BCC github repo in src/cc, with
> the clang specific bits inside of the frontends/clang subdirectory.
>
>> I'm not sure. Our target platform should be embedded devices like
>> smartphone.
>> Bringing full clang/llvm environment there is not acceptable.
> The artifact from the build process of BCC is a shared library, which
> has the clang/llvm .a embedded within them. It is not yet a single
> binary, but not unfeasible to make it so. The clang toolchain itself
> does not need to exist on the target. I have not attempted to
> cross-compile BCC to any architecture, currently x86_64 only.
>
> If you have more BCC specific questions not involving clang/llvm,
> perhaps you can ping Alexei/myself off of the llvm-dev list, in case
> this discussion is not relevant to them.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web