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

[Michael Eager]

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.

> 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.

> 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.

> 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.

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.

-- 
Michael Eager