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+# Adding Target Support
+
+LLDB supports many combinations of architecture, operating system and other
+system components. In this document we describe the considerations and
+requirements for porting LLDB to a combination of those things. Which we will
+refer to as a "target" within this document.
+
+:::{note}
+The terms `Target` and `Platform` are used throughout LLDB, often
+referring to a subset of what this document calls a "target".
+Unfortunately there is no more specific word to use here.
+
+This document provides some hints on implementation, but because every target
+is unique, we expect developers to learn about implementation from existing
+targets.
+
+This document starts with the end of the process, proposing your taret for
+upstream inclusion. It is worth reading through this even if that is not one of
+your goals, as you will have to tackle the same topics even in a downstream
+implementation.
+
+## Upstreaming Proposal Process
+
+You must send an RFC to the LLDB Discourse forum before upstreaming
+new target support. This RFC must be accepted in some form by the community
+before any changes can be merged upstream that are specific to your target.
+
+This RFC follows the normal rules of the LLVM community decision making
process.
+
+We require an RFC for target support upstreaming because:
+* We want ensure there is some level of public discussion on the topic.
+ It is important that details, even those obvious to the community at the
time,
+ are written down so that future readers may learn from it.
+
+ :::{note}
+ Assuming they were done in good faith, these discussions are not to be used
+ to personally criticise anyone after the fact. Situations change, people make
+ mistakes, and that is ok.
+
+* These discussions forms the basis for future proposals and the assessment
+ of those proposals. Every proposal will be different in some way, and by
+ contrast with the past we can do the best job assessing them.
+
+RFCs are not required to be:
+* In a set format. Make a logical argument in whatever way you think fit.
+* Exhaustively detailed. Include what you think is relevant and the community
+ will ask for the rest.
+* Answering all the same questions, or making all the same points, as previous
+ proposals. Compare your target with existing targets, but not everything
will,
+ or has to, apply to yours.
+* A commitment to the maximum theoretical level of support (more on this
later).
+ If your resources are limited, say so, and that will be taken into account
+ (and vice versa, committing to a lot of work does not guarantee acceptance).
+
+## Expectations Of Upstream Code
+
+Listed below are some examples of factors considered when considering accepting
+code upstream. These are examples and RFC authors are free to add their own,
+leave some out, or explain why they do not apply to their proposal.
+
+If you do use these points, they need to come with an answer and evidence to
+justify the answer. Rather than simply "yes this applies to my target".
+In other words, your proposal must stand alone without requiring readers to
read this document as well.
+
+The first set cover your motivation for your being upstream rather
+than on a fork:
+
+* Will it help you distribute an LLDB that includes this target support?
+ For example if there is an existing community for this
+ target, and how would they acquire LLDB?
+* Will it enable a wider community than your own forks would?
+ For example for it to be included in Linux distribution packaging.
+* Will it improve support for other targets by being there?
+ For example if we already support Operating System X on Architecture Y,
+ adding Architecture Z support may improve both as a side effect.
+* Will it help you keep your changes in sync?
+ For example if it involves fundamental changes to LLDB, or you have a very
+ small amount of maintenance resources in your community.
+* What other costs (or benefits) do you incur staying on a fork?
+ For example, your company might already have a fork.
+
+The next set are about whether you, your community, or the LLDB community, can
+adequately maintain the code upstream:
+
+* Who will be the maintainers for this target? Ideally there will be more than
+ one, who is present in the LLVM community and can be contacted
+ in a few different ways.
+* How often will it be tested, where, by whom and who will pay for it?
+* Who will address problems with it? Will it always be the named maintainers,
+ is it so common that anyone in upstream LLDB can deal with it, or perhaps
+ only employees of a specific company will be required to work on it.
+* When it breaks how easy will it be for the upstream LLDB community to
continue
+ their work without disruption?
+* If upstream contributors want to reproduce issues on your target, how can
+ they access it? Can it be emulated or virtualized? Does it require them to
+ sign a license? Do you offer access for open source projects? (and does that
+ include employees of other companies)
+
+## The Extent of Target Support
+
+Many of the factors mentioned above are on a spectrum. Not everyone can commit
+to spend maximum time, money, or effort on all those things. This is ok, not
+every target needs that and not every contributor is able to commit to that.
+
+We need you to help us understand the scope of your target's impact on LLDB. So
+below are some questions and sterotypical "small" and "big" answers.
+
+These are deliberately not "maximum" and "minimum" as that will vary between
+targets.
+
+* How many users will use LLDB with this target?
+ * Big: millions of developers worldwide.
+ * Small: you and a small community of target users.
+* How many changes will it require?
+ * Big: large changes to all parts of LLDB. On the level of the existing
+ code for C and C++ support.
+ * Small: small changes to enable existing support from LLVM, changes
+ to packet parsing (see the MSP430 case study below).
+* What parts and features of LLDB will work on, or with, this target?
+ * Big: `lldb`, `lldb-server` and advanced features like shared libraries.
+ * Small: just `lldb` and only basic features like continue, stop, reading
+ memory and registers.
+* How often is it tested?
+ * Big: per-commit testing of LLDB, following the upstream llvm-project.
+ * Small: per release of a downstream community or individual developer's
tools.
+* Who will maintain it?
+ * Big: there are several listed maintainers for this target, who are
+ employed by a company with significant investments in the target, to work
+ on the target.
+ * Small: there is a single maintainer listed for this target.
+* When it breaks, who will be affected?
+ * Big: every single developer and user of LLDB.
+ * Small: you and your target's community.
+* How do I reproduce a problem on this target?
+ * Big: all components are open source and can be run anywhere by using
+ simulations.
+ * Small: you contact the maintainer and they do it for you.
+* Who will fix problems?
+ * Big: the maintainers and a large group of contributors.
+ * Small: only the listed maintainer.
+
+## Target Support Case Studies
+
+These case studies are to give you starting points for your proposals. Consider
+how your target compares to these.
+
+### Apple Targets
+
+Apple targets have possibly the most extensive support in LLDB:
+* Changes from LLVM's main branch are continuously tested. Results are
accessible
+ publicly, and reported to all LLDB contributors.
+* Many employees contribute upstream and are maintainers for these targets.
+* Target specific problems are either solved by the upstream community with
+ maintainer input, or by the maintainers themselves.
+* Its specific features either do not impact other targets, or when they
+ do, they are designed and maintained in collaboration with the community.
+* Many people unrelated to Apple itself use Apple hardware, and hardware is
+ available at retail. So it is fairly easy to find someone who can reproduce
+ an issue.
+* LLDB gets a lot of secondary testing downstream, and exposure to Apple's
+ own developer community.
+
+### FreeBSD
+
+FreeBSD is an example of quite self contained support, managed by the
project's community.
+
+* FreeBSD is open source and available to anyone to build, modify, run on
+ hardware, emulate or virtualize.
+* FreeBSD support is mostly in the native target parts of `lldb-server`,
+ so issues with it rarely impact any other target.
+* It is quite similar to Linux, so it does not cause large scale changes to
+ fundamental assumptions LLDB makes.
+* It has at least one maintainer and sometimes contributions to upstream LLDB
+ from the FreeBSD community.
+* Issues are solved by the maintainer and the FreeBSD community.
+* LLDB on FreeBSD is tested each time FreeBSD updates the version used in the
+ base system, which is roughly per upstream LLVM release.
+
+### Linux
+
+Linux is an example where the Linux community as a whole does not do all the
work
+of Linux support. Some architectures have a wide contributor base and others
+have company specific contributors.
+
+* Linux is open source, freely available, to be installed on hardware,
virtualised,
+ emulated, and so on. Most problems can be reproduced in more than one way.
+* Continuous testing for many supported architectures, often supported by the
+ architecture vendor.
+* Remaining architectures are tested per release, or every other release.
+ Depends on the update schedule of the Linux distributions.
+* Support for a new architecture is quite easy to isolate, so problems do not
+ impact other Linux architectures, or other operating systems.
+* Linux's popularity and therefore it's choice of standards means it informs
+ many core features of LLDB.
+* It has more than one listed maintainer and many contributors.
+ Issues are resolved by the community.
+
+### MSP430
+
+MSP430 is handled as a bare metal (no operating system) target in LLDB. So it
+is the most minimal example.
+
+* LLVM already has MSP430 support, changes to enable it in LLDB were minimal.
+* It does not use `lldb-server`, and `lldb` only required minor adjustments to
+ be compatible with the commonly used debug server.
+* An ABI plugin was added, which is isolated to MSP430 only.
+* It is not systematically tested anywhere, but gets some use by MSP430
developers.
+* It has no documented maintainer.
+* We rely on users to report issues with it, and would likely guide them to fix
+ them themselves.
+
+## Components Of For Target Support
+
+This is a very high level view, we recommend you combine this with reading the
+changes done for recently added targets, as each target is going to be slightly
+different.
+
+Assuming that:
+* You want to support a combination of a new architecture and a new operating
+ system.
+* LLVM already supports your architecture and operating system.
+* You want to port most of the LLDB features, which means porting both `lldb`
+ and `lldb-server`.
+
+Then this is a list of components you will need to write. If your target is
similar
+to others you may be able to reuse existing components and we encourage you to
do
+so.
+
+First the components concerned with operating systems:
+
+* A `Platform` plugin. For example `PlatformLinux`.
+* A `HostInfo` plugin. For example `HostInfoLinux`.
+* A native process plugin (native means it runs on your target). For example
`NativeProcessLinux`.
+* Signal information. For example `LinuxSignals`.
+* A dynamic loader plugin. For example `DynamicLoaderPOSIXDYLD`.
+* An object file plugin. For example `ObjectFileELF`.
+
+Then the components concerned with architecture (though the distinction is
often
+inexact):
+
+* ABI plugin. For example `ABISysV_arm64`.
+* Architecture plugin. For example `ArchitectureAArch64`.
+* Register definitions and register context (a context is a collection of
registers).
+ For example `NativeRegisterContextLinux_arm64`.
+* Instruction emulation. Most targets need a small amount for unwind purposes,
+ but if you lack hardware single step you will need a lot more. For example
----------------
DavidSpickett wrote:
I've gone with "many times more". Just trying to show some rough upper and
lower bounds here.
> I think producing a non-exhaustive list of examples would be helpful here.
I'm not sure if you mean that the current list is that type of list, or you
want to see it expanded. Can you give an example of a thing you'd like to see
here?
Perhaps you mean specifically hardware features of the target that when present
save or create a lot of work for the debugger to do. E.g. hardware single step
- less work, delay slots - more work.
https://github.com/llvm/llvm-project/pull/207166
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