[lldb-dev] Proposal: How to modify lldb for non 8-bit bytes for kalimba processors
Todd Fiala
tfiala at google.com
Fri Aug 29 09:17:19 PDT 2014
> "You can't possibly do that, so many other architectures have 8-bit
bytes, and so this proposal would make them harder to enhance, for the
benefit of (currently) just kalimba"
>From a design principle, I would probably vote for having non-8-bit-bytes
added as a layer on top of memory accesses that is only used for systems
that need it. I'd strongly prefer not to have the generality of n-bit (or
multiple of 8-bit bytes) permute throughout the lldb code base. I can see
that becoming challenging from a maintainability standpoint (testing, code
complexity, etc.)
I could imagine a layer you build on top of memory accesses that knows how
to run out and grab the right underlying cpu_arch-specific bytes to satisfy
the request. And only wiring that in to low level memory access maybe in
the memory read/write virtual methods (like Process::DoReadMemory() and
friends).
That's just my take. Maintainability and testability being the key driver
here.
On Fri, Aug 29, 2014 at 5:11 AM, Matthew Gardiner <mg11 at csr.com> wrote:
> Based on some recent investigation, it looks as if I won't need to modify
> the CoreDefinition structure of ArchSpec.cpp. In a local change, I've added
> specification for the kalimba variants in the SubArchType of llvm::Triple.
> So it's now possible for me to implement
>
> uint32_t ArchSpec::GetCodeByteSize() const
> uint32_t ArchSpec::GetDataByteSize() const
>
> by inspection of subarch field of the triple contained in ArchSpec.
>
> However, I'd still appreciate some feedback on a more conceptual level
> regarding this proposal.
>
> thanks
> Matt
>
> Matthew Gardiner wrote:
>
>> Hi folks,
>>
>> One of the challenges that I need to resolve regarding debugging kalimba
>> processors, is that certain variants have different notions of the size (in
>> bits) of a byte, compared to a lot of more mainstream processors. What I'm
>> referring to is the size of a minimum addressable unit, when the processor
>> accesses memory. For example, on a kalimba architecture version 3, a "byte"
>> (minimum addressable unit) from the data bus is 24-bits, so if the
>> processor reads from address 8001 it reads 24-bits, and from address 8002
>> the next 24-bits are read, and so on... (this also means that for this
>> variant a char, int, long, pointer are 24-bits in size). For kalimba
>> architecture version 4, however, we have the minimum addressable unit being
>> 8-bits, and correspondingly more "conventional" sizes for primitive types.
>>
>> I imagine that this will effect the kalimba lldb port is various ways.
>> The most obvious one, and hence the one I'd like to solve first, is that
>> way in which raw memory read/write are implemented. As an example when I
>> ask lldb to read 4 "bytes" (addressable units worth of data) from a kalimba
>> with 8-bit bytes I expect to see this:
>>
>> (lldb) memory read --count 4 0x0328
>> 0x00000328: 00 07 08 08 ....
>> (lldb)
>>
>> However if target processor has 24-bit bytes then I expect the same query
>> to yield the following answer:
>>
>> (lldb) memory read --count 4 0x0328
>> 0x00000328: 000708 080012 095630 023480
>> ....
>> (lldb)
>>
>> Just considering the above scenario leads me to believe that my first
>> challenge is arranging for the remote protocol implementation (currently
>> Process/gdb-remote et al) to assume Nx host bytes (N being a
>> target-specific value) for each target byte accessed, and for the memory
>> read and formatting code (above) to behave correctly, given the discrepancy
>> between host and target byte sizes. I guess I'll see many other challenges
>> - for example, frame variable decode, stack unwind etc. (but since *those*
>> challenges require work on clang/llvm backend, and CSR have no llvm person
>> yet, I want to concentrate on raw memory access first...)
>>
>> For an added complication (since kalimba is a harvard architecture)
>> certain kalimba variants have differing addressable unit sizes for memory
>> on the code bus and data bus. Kalimba Architecture 5 has 8-bit addressable
>> code, and 24-bit addressable data...
>>
>> My initial idea for how to start to address the above challenge is to
>> augment the CoreDefinition structure in ArchSpec.cpp as follows:
>>
>> struct CoreDefinition
>> {
>> ByteOrder default_byte_order;
>> uint32_t addr_byte_size;
>> uint32_t min_opcode_byte_size;
>> uint32_t max_opcode_byte_size;
>> + uint32_t code_byte_size;
>> + uint32_t data_byte_size;
>> llvm::Triple::ArchType machine;
>> ArchSpec::Core core;
>> const char * const name;
>> };
>>
>> Where code_byte_size and data_byte_size would specify the size in host
>> (8-bit) bytes the sizes of the minimum addressable units on the referenced
>> architectures. So, e.g.
>> For kalimba 3, with 24-bit data bytes and 32-bit code bytes we'd have
>> data_byte_size=3 and code_byte_size=4
>> For kalimba 4, with 8-bit data bytes and 8-bit code bytes we'd have
>> data_byte_size=1 and code_byte_size=1
>>
>> So, then I'd update the g_core_definitions array within ArchSpec.cpp
>> accordingly, such that all non-kalimbas would have 1 as the setting for the
>> new datas and the kalimba entries would have those fields made to match the
>> architectures.
>>
>> The ArchSpec class would then require the following accessors: uint32_t
>> GetCodeByteSize() and uint32_t GetDataByteSize(); to supply client code
>> with the required hints to correctly implement memory accesses.
>>
>> My next plan would be to "massage" the code in the execution flow from an
>> (lldb) memory read invocation through to the gdb-remote comms until I see
>> the memory read examples I illustrated above, working for 8-bit and 24-bit
>> data kalimba targets.
>>
>> I'd appreciate all comments and opinions as to what I've described above
>> from the lldb community. Basically, I'm curious as to what people think of
>> the whole concept, e.g.
>>
>> "You can't possibly do that, so many other architectures have 8-bit
>> bytes, and so this proposal would make them harder to enhance, for the
>> benefit of (currently) just kalimba"
>> "Yes, that's a good idea, lldb can accommodate the most unusual of
>> architectures"
>>
>> And I'm also interested in technical comments, e.g. should an instance of
>> CoreDefinition be added to ArchSpec, or is just adding the extra byte-size
>> attributes sufficient... or if anyone thinks that modifying gdb-remote is a
>> bad idea, and that I should be creating kalimba process abstractions (and
>> factor out the common code)?
>>
>> thanks
>> Matt
>>
>>
>>
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--
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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