On Thu, Jul 16, 2020 at 6:35 PM Michael Eager <ea...@eagercon.com> wrote:
> On 7/16/20 5:37 PM, David Blaikie wrote: > > > > > > On Thu, Jul 16, 2020 at 4:25 PM Michael Eager <ea...@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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