AArch64: remove seL4_ARM_PageDirectory and seL4_ARM_PageUpperDirectory

Author: Kent McLeod
Proposed: 2022-03-11

Summary

This RFC proposes to remove two AArch64 VSpace object types, seL4_ARM_PageDirectory and seL4_ARM_PageUpperDirectory. The functionality previously provided by these types will be provided by the existing seL4_ARM_PageTable object type. This allows for a simpler API and enables a smaller kernel implementation that will be easier to verify.

Motivation

This change is motivated by an ongoing verification project to verify the AArch64 design and implementation. Unified PageTable object types was attempted during the RISCV64 verification project and it successfully reduced verification complexity. By applying the same design strategy to Aarch64, the verification can become easier.

The change is also motivated by an reducing complexity of the kernel user API where there are less object and capability types and less different invocation-object pairs while maintaining the same functionality. The ability to use PageTable objects for any level of the page-table hierarchy can also make virtual address space resource management easier as objects are more interchangeable.

Guide-level explanation

This change only affects the AArch64 architecture, but affects every configuration.

This change requires removing the seL4_ARM_PageDirectory and seL4_ARM_PageUpperDirectory object types. This means that it will no longer be possible to create these objects from Untyped capabilities. To make up for this, seL4_ARM_PageTable objects will be able to be mapped into an seL4_ARM_VSpace object or other seL4_ARM_PageTable objects.

This change will require modifications to existing user code. Every usage of an seL4_ARM_PageDirectory or seL4_ARM_PageUpperDirectory will need to be updated to instead use a seL4_PageTable. This update should be reasonably direct as all three object types have the same kind of invocations, are the same size, and contain the same number of table entries as each other.

Reference-level explanation

The entire sections of the seL4_ARM_PageDirectory and seL4_ARM_PageUpperDirectory interfaces have been removed from the libsel4 API XML interface definitions. The kernel will no longer recognize them as objects.

It's possible to redefine all of the deleted constants and symbols in the libsel4 implementation using CPP macro definitions in a deprecated.h header file. This can allow migration with minimal manual updating. Manual updates may still be required in places where the programming language used expects each object type to have a different number such as in a C switch statement.

In addition to a patch to the kernel, there is a patch to seL4_libs showing the minimal set of changes required to migrate the seL4test and seL4bench applications to the new API.

Internally, the kernel will use a single pte_t datatype to represent all page table entries in the VSpace implementation. This allows for considerably less code duplication for mapping and unmapping operations as well as for object creation and revocation. This new implementation follows the design taken with the RISC-V architecture.

Drawbacks

The main drawback for this change is that is API breaking -- existing user space programs will require their sources and binaries to be updated to use the newer kernel version. However, this will only affect configurations that previously have not been verified which should already be expecting API breakages as part of a verification process.

Rationale and alternatives

As AArch64 will be the third 64-bit architecture to go through a verification process we are able to make decisions that are informed by the experiences of the x86_64 and RISCV64 projects. In particular, the reduction in effort that this type unification allows was directly seen when comparing the x86_64 project, which used a separate object type for each level of the page table hierarchy, to the RISCV64 project which uses a single type.

One change that has been made compared to the RISC-V design is using a different type for the seL4_ARM_VSpace object. This object serves as the root page table of the address space. The AArch64 virtual memory architecture allows the top level page table to be sized differently depending on the size of the address space. This object also supports additional invocations that can't be applied to intermediate page table objects and it is also likely to gain more invocations in the future.

The seL4 RFC Book