Re: [PATCH dwarves v2 2/2] btf: Support BTF_KIND_ENUM64

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On 6/27/22 3:30 PM, Andrii Nakryiko wrote:
On Wed, Jun 15, 2022 at 4:03 PM Yonghong Song <yhs@xxxxxx> wrote:

BTF_KIND_ENUM64 is supported with latest libbpf, which
supports 64-bit enum values. Latest libbpf also supports
signedness for enum values. Add enum64 support in
dwarf-to-btf conversion.

The following is an example of new encoding which covers
signed/unsigned enum64/enum variations.

   $cat t.c
   enum { /* signed, enum64 */
     A = -1,
     B = 0xffffffff,
   } g1;
   enum { /* unsigned, enum64 */
     C = 1,
     D = 0xfffffffff,
   } g2;
   enum { /* signed, enum */
     E = -1,
     F = 0xfffffff,
   } g3;
   enum { /* unsigned, enum */
     G = 1,
     H = 0xfffffff,
   } g4;
   $ clang -g -c t.c
   $ pahole -JV t.o
   btf_encoder__new: 't.o' doesn't have '.data..percpu' section
   Found 0 per-CPU variables!
   File t.o:
   [1] ENUM64 (anon) size=8
           A val=-1
           B val=4294967295
   [2] INT long size=8 nr_bits=64 encoding=SIGNED
   [3] ENUM64 (anon) size=8
           C val=1
           D val=68719476735
   [4] INT unsigned long size=8 nr_bits=64 encoding=(none)
   [5] ENUM (anon) size=4
           E val=-1
           F val=268435455
   [6] INT int size=4 nr_bits=32 encoding=SIGNED
   [7] ENUM (anon) size=4
           G val=1
           H val=268435455
   [8] INT unsigned int size=4 nr_bits=32 encoding=(none)

With the flag to skip enum64 encoding,

   $ pahole -JV t.o --skip_encoding_btf_enum64
   btf_encoder__new: 't.o' doesn't have '.data..percpu' section
   Found 0 per-CPU variables!
   File t.o:
   [1] ENUM (anon) size=8
         A val=4294967295
         B val=4294967295
   [2] INT long size=8 nr_bits=64 encoding=SIGNED
   [3] ENUM (anon) size=8
         C val=1
         D val=4294967295
   [4] INT unsigned long size=8 nr_bits=64 encoding=(none)
   [5] ENUM (anon) size=4
         E val=4294967295
         F val=268435455
   [6] INT int size=4 nr_bits=32 encoding=SIGNED
   [7] ENUM (anon) size=4
         G val=1
         H val=268435455
   [8] INT unsigned int size=4 nr_bits=32 encoding=(none)

In the above btf encoding without enum64, all enum types
with the same type size as the corresponding enum64. All these
enum types have unsigned type (kflag = 0) which is required
before kernel enum64 support.

Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
  btf_encoder.c     | 65 +++++++++++++++++++++++++++++++++++------------
  btf_encoder.h     |  2 +-
  dwarf_loader.c    | 12 +++++++++
  dwarves.h         |  4 ++-
  dwarves_fprintf.c |  6 ++++-
  pahole.c          | 10 +++++++-
  6 files changed, 79 insertions(+), 20 deletions(-)


Sorry for late review, I don't always catch up on emails from older
emails first :(

[...]

         size = BITS_ROUNDUP_BYTES(bit_size);
-       id = btf__add_enum(btf, name, size);
+       is_enum32 = size <= 4 || no_enum64;
+       if (is_enum32)
+               id = btf__add_enum(btf, name, size);
+       else
+               id = btf__add_enum64(btf, name, size, is_signed);
         if (id > 0) {
                 t = btf__type_by_id(btf, id);
                 btf_encoder__log_type(encoder, t, false, true, "size=%u", t->size);
         } else {
-               btf__log_err(btf, BTF_KIND_ENUM, name, true,
+               btf__log_err(btf, is_enum32 ? BTF_KIND_ENUM : BTF_KIND_ENUM64, name, true,
                               "size=%u Error emitting BTF type", size);
         }
         return id;
  }

-static int btf_encoder__add_enum_val(struct btf_encoder *encoder, const char *name, int32_t value)
+static int btf_encoder__add_enum_val(struct btf_encoder *encoder, const char *name, int64_t value,
+                                    bool is_signed, bool is_enum64, bool no_enum64)

It was quite confusing to see "is_enum64" and "no_enum64" as arguments
to the same function :)

I'll let Arnaldo decide for himself, but I think it would be cleaner
to pass such configuration switches as fields in struct btf_encoder
itself and just check such flags from relevant btf_encoder__add_xxx()
functions. Such flags are global by nature, so it seems fitting.

This will require an additional struct definition. It should work but I will wait for Arnaldo's comments as well.


But other than that looks good to me.

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

  {
-       int err = btf__add_enum_value(encoder->btf, name, value);
+       const char *fmt_str;
+       int err;
+
+       /* If enum64 is not allowed, generate enum32 with unsigned int value. In enum64-supported
+        * libbpf library, btf__add_enum_value() will set the kflag (sign bit) in common_type
+        * if the value is negative.
+        */
+       if (no_enum64)
+               err = btf__add_enum_value(encoder->btf, name, (uint32_t)value);
+       else if (is_enum64)
+               err = btf__add_enum64_value(encoder->btf, name, value);
+       else
+               err = btf__add_enum_value(encoder->btf, name, value);

[...]



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