Documentation: Add guidelines document for kernel development

Documentation/Kernel/DevelopmentGuidelines.md

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+# Kernel Development Patterns & Guidelines
+
+This document intends to guide the immediate and newcomer kernel developer when creating,
+modifying and removing Kernel code.
+Please read all of this document if you intend to send pull requests, as well as the [general contributing guidelines](../../CONTRIBUTING.md)
+and [patterns](../Patterns.md) for the entire codebase.
+
+This document was composed as a result of ideas, experience and a general vision of what
+the Kernel could become in the prosperous future of the project.
+
+## Out of memory handling
+
+Maybe one of the most important issues we have to solve in kernel code is when OOM (Out of memory)
+condition occurs - simply put, a new allocation request has failed due to various reasons -
+the allocation request was too much "greedy" and couldn't be satisfied, or simply we can't allocate more physical RAM pages
+to whoever that requested them.
+
+**The proper solution to this is to always use the `TRY()` semantics together with
+appropriate `adopt_*` function (either for `OwnPtr` or `RefPtr`).**
+
+```cpp
+#include <AK/Try.h>
+#include <AK/OwnPtr.h>
+
+...
+
+auto new_object = TRY(adopt_nonnull_own_or_enomem(new (nothrow) Object(...)));
+```
+
+In case of failure, the above code will simply propagate `ENOMEM` code to the caller, up to the syscall entry code,
+so the userland program could know about the situation and act accordingly.
+
+An exception to this is when there's simply no way to propagate the error code to the userland program.
+Maybe it's a `ATAPort` (in the IDE ATA code) that asynchronously tries to handle reading data from the harddrive,
+but because of the async operation, we can't send the `errno` code back to userland, so we what we do is
+to ensure that internal functions still use the `ErrorOr<>` return type, and in main calling function, we use
+other meaningful infrastructure utilties in the Kernel to indicate that the operation failed.
+
+## We don't break userspace - the SerenityOS version
+
+We don't break userspace. However, in contrast to the Linux vision on this statement,
+we don't care about ABI/API breakage between the userland and the kernel. **What we do care
+about is a possible incident when a Kernel change does introduce a misbehave in userland, and Userland was not
+appropriately considered to ensure this does not happen.**
+
+Many internal changes in the Kernel don't affect userland - for example, a new shiny driver
+for super-fast storage devices, is not something that will likely break userland, because the
+proper abstractions have already put in place, so the userland simply does not care about
+the specifics about each `StorageDevice` in the kernel, as long as it properly implements the
+known interfaces.
+
+However, some kernel changes, mainly ABI/API changes between userland and the kernel, in the
+syscall handling layer, will break userland unless it's properly handled beforehand.
+The proper solution in git terms is to ensure that both "offending" kernel changes and the appropriate
+userland changes to accommodate the kernel changes are in the same commit, so we still keep the rule that
+each git commit is bisectable by itself.
+
+**It's expected that changes to the Kernel will be tested with userland utilities to ensure the changes
+are not creating any misbehaves in the userland functionality.**
+
+Even more strictier than what has been said above - we don't remove functionality unless it's absolutely
+clear that nobody uses that functionality. Even when it's absolutely clear that nobody uses some kind
+of kernel functionality, it could still be useful to think about how to make it more available and usable
+to the SerenityOS project community.
+Again, such removal should happen according to what has been mentioned in terms of git handling.
+
+## Each kernel feature should be backed by a userland usecase
+
+In contrast to the previous guideline, this guideline is clearly about the healthy growth of Kernel -
+we don't bloat the Kernel for things we don't need. For example, in the early days
+of the project, there was a floppy driver in the Kernel and it got removed because
+nobody used it. Similarly, when an Intel AC97 soundcard driver was introduced, the SB16
+soundcard driver was removed shortly afterwards. **We simply don't have interest in supporting
+hardware that nobody will use, or a kernel feature that doesn't make sense to most people.**
+
+## Proper locking
+
+The [AHCI locking document](AHCILocking.md) describes our locking patterns thoroughly.
+Still, it's very important to understand we do care about SMP (Symmetric Multiprocessing),
+so proper locking is one of the top priorities in the kernel development mindset.
+
+**The general rule is that we should not acquire a `Mutex` after taking a `Spinlock`.
+Taking a `Spinlock` after another is generally considered fine, as long as they are always
+taken in the same order, to prevent deadlocks.**
+
+To ensure we do this properly, the `MutexProtected<>` and `SpinlockProtected<>` C++ containers
+have been introduced in the kernel to ensure that locking is done on particular shared data objects,
+so it's preferable to use these containers instead of a "random" spinlock as class member.
+
+## Proper, clean and meaningful syscall userland interfaces
+
+As at the time of writing this document, the syscall table is generally quite stable.
+This happens to be that way because the syscalls are well-defined, backed by good-known POSIX interfaces.
+**Suggestions/patches to add syscalls should be examined strictly, because generally-speaking it's the "last resort"
+we should choose from other Unix interfaces that are available to us.**
+
+Because there's no definitive "yes" or "no" for all cases, expect that a discussion will be taking
+place in your pull request, in case that you do introduce a new syscall in the Kernel.
+
+For example, say that one wants to add a new driver for the Storage subsystem, then
+we already have the proper abstractions in place, so the new specific `StorageDevice` will be registered
+as like any other `StorageDevice`, therefore it will be exposed in the `/dev` directory and regular
+`write`, `open`, `read`, `ioctl` syscalls will be usable immediately.
+Therefore, there's no need for a special syscall to handle the new hardware, because
+the already-existing syscalls are sufficient.
+
+**We should also refrain from architecture-specific syscalls as much as possible. Linux had them
+in the past and many of them were removed eventually.**
+
+## Security measures
+
+We, as the SerenityOS project, take seriously the concept of security information.
+Many security mitigations have been implemented in the Kernel, and are documented in a
+[different file](../../Base/usr/share/man/man7/Mitigations.md).
+As kernel developers, we should be even more strictier on the security measures being
+taken than the rest of system.
+One of the core guidelines in that aspect **is to never undermine any security measure
+that was implemented, at the very least.**
+
+It's also very nice and generous if one decides to improve on a security measure,
+as long as it doesn't hurt other security measures.
+
+We also consider performance metrics, so a tradeoff between two mostly-contradictive metrics
+is to be discussed when an issue arises.
+
+## Documentation
+
+As with any documentation, it's always good to see more of it, either with a new manual page,
+or a kernel concept being described in the `Documentation/Kernel` repository directory so other
+developers can understand it.
+There's no well-defined template to use when writing a documentation, but it is expected
+at the very least to have an opening paragraph about the topic so others can understand
+what the document is about.