Message-ID: <CA+icZUV=-NmFtF9RQTRnbwBUiaPnroiSwyv-9RxA-3-nrgQ_rQ@mail.gmail.com>
Date:   Sat, 6 Feb 2021 20:28:59 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Mark Wieelard <mark@...mp.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        dwarves@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        bpf@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Jan Engelhardt <jengelh@...i.de>,
        Domenico Andreoli <cavok@...ian.org>,
        Matthias Schwarzott <zzam@...too.org>,
        Andrii Nakryiko <andriin@...com>,
        Paul Moore <paul@...l-moore.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Daniel P. Berrangé <berrange@...hat.com>,
        Tom Stellard <tstellar@...hat.com>
Subject: Re: ERROR: INT DW_ATE_unsigned_1 Error emitting BTF type

On Sat, Feb 6, 2021 at 8:22 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Sat, Feb 6, 2021 at 8:17 PM Yonghong Song <yhs@...com> wrote:
> >
> >
> >
> > On 2/6/21 10:10 AM, Sedat Dilek wrote:
> > > On Sat, Feb 6, 2021 at 6:53 PM Yonghong Song <yhs@...com> wrote:
> > >>
> > >>
> > >>
> > >> On 2/6/21 8:24 AM, Mark Wieelard wrote:
> > >>> Hi,
> > >>>
> > >>> On Sat, Feb 06, 2021 at 12:26:44AM -0800, Yonghong Song wrote:
> > >>>> With the above vmlinux, the issue appears to be handling
> > >>>> DW_ATE_signed_1, DW_ATE_unsigned_{1,24,40}.
> > >>>>
> > >>>> The following patch should fix the issue:
> > >>>
> > >>> That doesn't really make sense to me. Why is the compiler emitting a
> > >>> DW_TAG_base_type that needs to be interpreted according to the
> > >>> DW_AT_name attribute?
> > >>>
> > >>> If the issue is that the size of the base type cannot be expressed in
> > >>> bytes then the DWARF spec provides the following option:
> > >>>
> > >>>       If the value of an object of the given type does not fully occupy
> > >>>       the storage described by a byte size attribute, the base type
> > >>>       entry may also have a DW_AT_bit_size and a DW_AT_data_bit_offset
> > >>>       attribute, both of whose values are integer constant values (see
> > >>>       Section 2.19 on page 55). The bit size attribute describes the
> > >>>       actual size in bits used to represent values of the given
> > >>>       type. The data bit offset attribute is the offset in bits from the
> > >>>       beginning of the containing storage to the beginning of the
> > >>>       value. Bits that are part of the offset are padding.  If this
> > >>>       attribute is omitted a default data bit offset of zero is assumed.
> > >>>
> > >>> Would it be possible to use that encoding of those special types?  If
> > >>
> > >> I agree with you. I do not like comparing me as well. Unfortunately,
> > >> there is no enough information in dwarf to find out actual information.
> > >> The following is the dwarf dump with vmlinux (Sedat provided) for
> > >> DW_ATE_unsigned_1.
> > >>
> > >> 0x000e97e9:   DW_TAG_base_type
> > >>                   DW_AT_name      ("DW_ATE_unsigned_1")
> > >>                   DW_AT_encoding  (DW_ATE_unsigned)
> > >>                   DW_AT_byte_size (0x00)
> > >>
> > >> There is no DW_AT_bit_size and DW_AT_bit_offset for base type.
> > >> AFAIK, these two attributes typically appear in struct/union members
> > >> together with DW_AT_byte_size.
> > >>
> > >> Maybe compilers (clang in this case) can emit DW_AT_bit_size = 1
> > >> and DW_AT_bit_offset = 0/7 (depending on big/little endian) and
> > >> this case, we just test and get DW_AT_bit_size and it should work.
> > >>
> > >> But I think BTF does not need this (DW_ATE_unsigned_1) for now.
> > >> I checked dwarf dump and it is mostly used for some arith operation
> > >> encoded in dump (in this case, e.g., shift by 1 bit)
> > >>
> > >> 0x000015cf:   DW_TAG_base_type
> > >>                   DW_AT_name      ("DW_ATE_unsigned_1")
> > >>                   DW_AT_encoding  (DW_ATE_unsigned)
> > >>                   DW_AT_byte_size (0x00)
> > >>
> > >> 0x00010ed9:         DW_TAG_formal_parameter
> > >>                         DW_AT_location    (DW_OP_lit0, DW_OP_not,
> > >> DW_OP_convert (0x000015cf) "DW_ATE_unsigned_1", DW_OP_convert
> > >> (0x000015d4) "DW_ATE_unsigned_8", DW_OP_stack_value)
> > >>                         DW_AT_abstract_origin     (0x00013984 "branch")
> > >>
> > >> Look at clang frontend, only the following types are encoded with
> > >> unsigned dwarf type.
> > >>
> > >>     case BuiltinType::UShort:
> > >>     case BuiltinType::UInt:
> > >>     case BuiltinType::UInt128:
> > >>     case BuiltinType::ULong:
> > >>     case BuiltinType::WChar_U:
> > >>     case BuiltinType::ULongLong:
> > >>       Encoding = llvm::dwarf::DW_ATE_unsigned;
> > >>       break;
> > >>
> > >>
> > >>> not, can we try to come up with some extension that doesn't require
> > >>> consumers to match magic names?
> > >>>
> > >
> > > You want me to upload mlx5_core.ko?
> >
> > I just sent out a patch. You are cc'ed. I also attached in this email.
> > Yes, it would be great if you can upload mlx5_core.ko so I can
> > double check with this DW_ATE_unsigned_160 which is really usual.
> >
>
> Yupp, just built a new pahole :-).
> Re-building linux-kernel...
>
> Will upload mlx5_core.ko - need zstd-ed it before.
>

Hmm, I guess you want a mlx5_core.ko with your patch applied-to-pahole-1.20 :-)?

> - Sedat -
>
> > >
> > > When looking with llvm-dwarf for DW_ATE_unsigned_160:
> > >
> > > 0x00d65616:   DW_TAG_base_type
> > >                 DW_AT_name      ("DW_ATE_unsigned_160")
> > >                 DW_AT_encoding  (DW_ATE_unsigned)
> > >                 DW_AT_byte_size (0x14)
> > >
> > > If you need further information, please let me know.
> > >
> > > Thanks.
> > >
> > > - Sedat -
> > >

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