[Dwarf-Discuss] Segment selectors for the range list table.

[David Blaikie]

On Thu, Jul 16, 2020 at 6:35 PM Michael Eager <eager at eagercon.com> wrote:

> On 7/16/20 5:37 PM, David Blaikie wrote:
> >
> >
> > On Thu, Jul 16, 2020 at 4:25 PM Michael Eager <eager at eagercon.com
>
> > The language used to describe segmented addressing in DW_AT_segment
> > reads to me like the same language used to describe segmented addresses
> > in debug_aranges - it reads to me like they refer to the same concept.
> > What aspect of the wording do you find distinguishes between these two
> > discussions on segmented addresses?
>
> They refer to the same segmented addresses.
>
> There are not two discussions of segmented addresses in the DWARF spec.
>
> >     Reread my previous comment about the meaning of segment as used in
> >     DW_AT_segment.   You apparently want it to mean something different.
> >
> >
> > There seems to be some significant disconnect between how we're each
> > understanding these concepts - could you help me understand/perhaps use
> > different language that might help me understand/connect with your
> > reading/understanding here? I'm trying my best to understand what the
> > spec is saying/how to interpret it.
>
> DW_AT_segment was intended to support addressing schemes like that used
> by x86 models.  The description is somewhat generalized, but that is the
> core of the concept.  If Intel used different terminology for their
> scheme, we would have used a different name.  In their usage, a
> segmented address maps (many-to-one) on a linear address space.
>
> Whether DW_AT_segment can be used to support a more general concept of
> disjoint address spaces is unclear.  AFAIK, no one has used it for this
> purpose.  DW_AT_address_class has been used, as I understand it, but I'm
> not familiar with these architectures.
>
> > You've mentioned they can be used - but I'm still pretty confused by how
> > they would be used to achieve that result. Do you happen to know of an
> > implementation that uses them in this way/any examples of DWARF using
> > the feature? I think that'd be realyl helpful to ground the discussion
> > with concrete examples.
>
> I've given you a concrete example: x86.
>

I was hoping for steps to produce DWARF that uses the feature from an
existing producer, to see how that producer interpreted the wording we're
discussing.

I'll see if I can make GCC do this, but I'm pretty unfamiliar with GCC for
non-host architectures - wouldn't mind any more specific tips if you have
them.

> My reading still seems to indicate that all the dwarf sections are meant
> > to use segment-relative addresses (all the wording for address size says
> > it should be the segment-relative address size, not an address in an
> > alternative linear address space)
>
> The majority (that is, all but one) of architectures use flat address
> spaces and have segment size = 0.
>
> The fact that DWARF supports an architecture which has a non-zero
> segment size should not be taken (as you appear to) to imply that all
> architectures must be somehow compelled to use non-zero segment size.
> The availability of a feature does not mandate its use.
>

Sorry, I didn't mean to imply that. I think what I meant to imply is that
it seems like if a producer is using segmented addresses in one part of
their output, they may want or need to do so in all sections.

But yeah, fully don't expect to need/use segmented addressing at all in
most/current producers.

> It sounds like you're suggesting that an implementation may choose on a
> > per-section basis whether to use segmented addressing (because this
> > assumes an existing alternative linear addressing per x86) - and that
> > some sections /require/ the use of that linear addressing mode. Is that
> > the idea?
>
> An x86 memory model implementation might use segmented addressing in the
> .debug_info section, but not in the .debug_line section.
>
> Given the aliasing in x86 segmented memory, I'm not sure how it could be
> supported in the line table.
>

Yep, pretty confused about that - hopefully with a concrete example this'll
be more clear to me.

> But the segmented addressing section seems to say "addresses are
> > specified as offsets within a given segment rather than as locations
> > within a single flat address space" - sounds like it's talking about
> > systems where there is no flat address space, in which case the sections
> > requiring a linear addressing would present a problem when it comes to
> > rendering them in DWARF.
>
> You are reading more into the text than is there.  Indeed, the example
> given does have a linear address space.
>
> If you want a concrete example, x86.   If you want to postulate an
> architecture which cannot be described in DWARF, have at it, but it's
> not a fruitful or illuminating discussion.
>
> >     DWARF uses the same description for segmented addresses almost
> >     everywhere that an address is used.  This is for consistency.  The
> >     meaning is the same everywhere.
> >
> >
> > If the meaning is the same everywhere, then it seems strange that it's
> > not supported everywhere (such as debug_rnglist/ranges) - the meaning
> > used in 2.12 seems to talk about a possible architecture that can only
> > be addressed via segmented addressing, without an alternative linear
> > address space - yet there are bits of DWARF where support for such an
> > architecture seems to be missing/incomplete.
>
> Section 2.12 does not mention or imply any possible or hypothetical
> architecture  which has the restrictions you suggest.  It mentions the
> same one I've repeated multiple times.
>

Oh, it mentions that for address spaces - it doesn't seem to mention any
concrete example for segment selectors.


> Whether DWARF can describe the kind of architecture which you
> hypothesize is moot.  When you have such an architecture and wish to
> support it in DWARF, then you would have something real to discuss.
>
> > I'd love to see some concrete examples of this sort of DWARF if you know
> > of any producer that uses this feature.
>
> Check if GCC still generates  code for x86.  Or find an old version.
>

Will see what I can muster up.

To answer the Xing's original question:

--------
The DWARFv5 spec mentioned that there might be segment selectors in
the range list entries and when the segment_selector_size is 0, the
segment selectors are omitted from the range list entries. However, it
didn't mention how the segment selector should be encoded when the
segment_selector_size isn't 0. Can anyone help me figure it out?
Thanks a lot!

7.28 (page 243)
The segment size is given by the segment_selector_size field of the
header, and the address size is given by the address_size field of the
header. If the segment_selector_size field in the header is zero, the
segment selector is omitted from the range list entries.
------

Sounds like the answer you're suggesting is: This wording is
unnecessary/misleading - that there is no use of segment selectors (even
when the header specifies a non-zero segment selector size) in
debug_rnglists (& presumably similarly in debug_loclists). Same as was the
case in DWARFv4?
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