On 04/19/2018 08:22 PM, Jim Ingham wrote:


On Apr 19, 2018, at 10:54 AM, Greg Clayton <clayb...@gmail.com> wrote:



On Apr 19, 2018, at 10:35 AM, Jim Ingham <jing...@apple.com> wrote:



On Apr 19, 2018, at 9:44 AM, Greg Clayton via lldb-dev 
<lldb-dev@lists.llvm.org> wrote:



On Apr 19, 2018, at 6:51 AM, Zdenek Prikryl via lldb-dev 
<lldb-dev@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.


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

So, does it make sense to start with lldb::addr_t replacement? In other words, replace all instances of lldb::addr_t with Address. It'd be the first step and first patch towards to the ability to extend it in the future, right?

Zdenek




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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