This patch extends the protection domain framework with an additional
plugin to use Task-State Segment (TSS) structures to offload much of
the work of switching protection domains to the CPU. This can save
space compared to paging, since paging requires two 4KiB page tables
and one 32-byte page table plus one whole-system TSS and an additional
32-byte data structure for each protection domain, whereas the
approach implemented by this patch just requires a 128-byte data
structure for each protection domain. Only a small number of
protection domains will typically be used, so
n * 128 < 8328 + (n * 32).
For additional information, please refer to cpu/x86/mm/README.md.
GCC 6 is introducing named address spaces for the FS and GS segments
[1]. LLVM Clang also provides address spaces for the FS and GS
segments [2]. This patch also adds support to the multi-segment X86
memory management subsystem for using these features instead of inline
assembly blocks, which enables type checking to detect some address
space mismatches.
[1] https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html
[2] http://llvm.org/releases/3.3/tools/clang/docs/LanguageExtensions.html#target-specific-extensions
This patch implements a simple, lightweight form of protection domains
using a pluggable framework. Currently, the following plugin is
available:
- Flat memory model with paging.
The overall goal of a protection domain implementation within this
framework is to define a set of resources that should be accessible to
each protection domain and to prevent that protection domain from
accessing other resources. The details of each implementation of
protection domains may differ substantially, but they should all be
guided by the principle of least privilege. However, that idealized
principle is balanced against the practical objectives of limiting the
number of relatively time-consuming context switches and minimizing
changes to existing code.
For additional information, please refer to cpu/x86/mm/README.md.
This patch also causes the C compiler to be used as the default linker
and assembler.
