[lldb-dev] Advice on architectures with multiple address spaces

Fri Apr 20 05:36:13 PDT 2018

On 04/19/2018 08:22 PM, Jim Ingham wrote:
>> On Apr 19, 2018, at 10:54 AM, Greg Clayton <clayborg at gmail.com> wrote:
>>
>>
>>
>>> On Apr 19, 2018, at 10:35 AM, Jim Ingham <jingham at apple.com> wrote:
>>>
>>>
>>>
>>>> On Apr 19, 2018, at 9:44 AM, Greg Clayton via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>>>
>>>>
>>>>
>>>>> On Apr 19, 2018, at 6:51 AM, Zdenek Prikryl via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Hi lldb developers,
>>>>>
>>>>> I've been researching using lldb + gdbserver stub that is based on Harvard architecture with multiple address spaces (one program, multiple data). The commonly adopted approach is that everything is mapped to a single "virtual" address space. The stub reads/writes from/to the right memory based on the "virtual" addresses. But I'd like to use real addresses with address space id instead. So, I've started looking at what has to be changed.
>>>>>
>>>>> I've enhanced read/write commands (e.g. memory read --as <id> ...) and RSP protocol (new packet) so that the stub can read/write properly. That wasn't that complicated.
>>>> It might be nice to add a new RSP protocol packet that asks for the address space names/values:
>>>>
>>>> qGetAddressSpaces
>>>>
>>>> which would return something like:
>>>>
>>>> 1:text;2:data1,3:data2
>>>>
>>>> or it would return not supported. If we get a valid return value from qGetAddressSpaces, then it enables the use of the new packet you added above. Else it defaults to using the old memory read functions.

Sounds good to me. I would return more information though. For instance, 
you can have a code address space where a 32bit byte is used and data 
address space where an 8bit byte is used. Some support for this is 
already in LLDB, although it's not tied to address spaces, but to an 
architecture.

>>>>
>>>>
>>>>> Now I've hit an issue with expressions (LLVMUserExpression.cpp) and local variables (DWARFExpressions.cpp). There is a lot of memory read/write functions that take just an address argument. Is the only way to go to patch all these calls? Has anybody solved it differently?
>>>> My quick take is that any APIs that take just a lldb::addr_t would need to take something like:
>>>>
>>>> struct SpaceAddress {
>>>> static constexpr uint32_t kNoSpace = 0;
>>>> lldb::addr_t addr;
>>>> uint32_t space;
>>>> };
>>>>
>>> I'm curious why you are suggesting another kind of address, rather than adding this functionality to Address?  When you actually go to resolve an Address in a target with a process you should have everything you need to know to give it the proper space.  Then fixing the expression evaluator (and anything else that needs fixing) would be a matter of consistently using Address rather than lldb::addr_t.  That seems general goodness, since converting to an lldb::addr_t loses information.
>> If we accept lldb_private::Address in all APIs that take a lldb::addr_t currently, then we need to always be able to get to the target in case we need to add code to resolve the address everywhere. I am thinking of SpaceAddress as an augmented lldb::addr_t instead of a section + offset style address. Also, there will be addresses in the code and data that do not exist in actual sections. Not saying that you couldn't use lldb_private::Address. I am open to suggestions though. So your though it remove all API that take lldb::addr_t and use lldb_private::Address everywhere all the time?
> It has always bugged me that we have these two ways of specifying addresses.  Are there many/any places that have to resolve an Address to a real address in a process that don't have a Target readily available?  That would surprise me.  I would much rather centralize on one way than adding a third.
>
> Jim

I'd like to remove more ways of describing the same thing, so going with 
the Address() sounds better. Having said that, there are about 4k 
instances of lldb::addr_t in LLDB code base. Where to begin/how to split 
the work? :-)...

>
>
>>> Jim
>>>
>>>
>>>> We would need a default value for "space" (feel free to rename) that indicates the default address space as most of our architectures would not need this support. If we added a constructor like:
>>>>
>>>> SpaceAddress(lldb::addr_t a) : addr(a), space(kNoSpace) {}
>>>>
>>>> Then all usages of the APIs that used to take just a "lldb::addr_t" would implicitly call this constructor and continue to act as needed. Then we would need to allow lldb_private::Address objects to resolve to a SpaceAddress:
>>>>
>>>> SpaceAddress lldb_private::Address::GetSpaceAddress(Target *target) const;
>>>>
>>>> Since each lldb_private::Address has a section and each section knows its address space. Then the tricky part is finding all locations in the expression parser and converting those to track and use SpaceAddress. We would probably need to modify the allocate memory packets in the RSP protocol to be able to allocate memory in any address space as well.
>>>>
>>>> I didn't spend much time think about correct names above, so feel free to suggest alternate naming.
>>>>
>>>> Best advice:
>>>> - make things "just work" to keep changes to a minimum and allowing lldb::addr_t to implicitly convert to a SpaceAddress easily
>>>> - when modifying RSP, make sure to check for existence of new feature before enabling it
>>>> - query for address space names so when we dump SpaceAddress we can show something that means something to the user. This means we would need to query the address space names from the current lldb_private::Process for display.
>>>>
>>>> Submitting might go easier if we break it down into chunks:
>>>> 1 - add SpaceAddress and modify all needed APIs to use it
>>>> 2 - add ProcessGDBRemote changes that enable this support
>>>>
>>>> It will be great to support this as a first class citizen within LLDB. You might ask the Hexagon folks if they have done anything in case they already support this is some shape or form.
>>>>
>>>> Greg Clayton
>>>>
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