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

Thu May 17 04:38:23 PDT 2018

The Address class may be suitable for the higher layers of lldb, but I
don't think the it can ever be a blanket replacement for lldb::addr_t. It
has way too much smartness built-in. We use addr_t in a lot of places that
don't/shouldn't care about Targets, ExecutionContexts or Sections. All of
lldb-server is one of those places, but this is also true for any low-level
operation which only wants to work with real (virtual) addresses in the
process address space.

On the other hand, replacing addr_t with a lighweight struct which is just
adds some sort of an address space identifier seems like a useful thing,
and would go a long way towards bringing Harward architecture support to
lldb-server. (Note that we would still need an addr_t or something of that
form to name the type of the "address" member of the struct, but pretty
much all of the apis that currently take addr_t, could that the new struct
instead).

On Thu, 17 May 2018 at 12:01, Zdenek Prikryl via lldb-dev <
lldb-dev at lists.llvm.org> wrote:

> 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.
> >>>>
> >>>>
> >>>>> 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
> >>>>
> >>>> _______________________________________________
> >>>> lldb-dev mailing list
> >>>> lldb-dev at lists.llvm.org
> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev

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