Future work

Our effort to analyze and optimize performance, starting with the original taped-out Morello design, and now continuing on FPGA, remains at an early stage, with many next steps to take. These include:

  • Application of the “Benchmark ABI” and this approach to other workloads. This is an important step, as limitations on speculation for short function executions can have a disproportionate impact on certain workloads, and is also very sensitive to compiler and linker choices regarding code layout and inlining. Toolchain support for the Benchmark ABI was made available in December 2023 as part of the CheriBSD 23.11 release, which supports statically and dynamically linked binaries, as well as roughly 10K open-source packages, compiled to the Benchmark ABI to support third-party performance analysis on Morello.

  • Improving the completeness of the P128 compilation mode and ABI, which will allow us to better project future performance not just on the Morello design, but also other non-CHERI-enabled microarchitectures (and architectures). Specific goals here would include widening GOT entries and improving layout decisions around spilled stack entries of unknown type (e.g., callee-save registers). It will also be important to compare the sets of optimizations being applied to baseline aarch64 (legacy), P128, and aarch64c (pure-capability) code in order to better understand potential performance comparisons. We have not yet reached a decision on supporting third-party users of the P128 compilation mode on CheriBSD due to the complexity of supporting the ABI.

  • Improving our understanding of security-performance tradeoffs and optimization opportunities around the pure-capability ABIs and code generation. For example, currently all access to global variables from pure-capability code is indirected through the GOT, whereas aarch64 is able to perform some accesses via the program counter. Indirecting via the GOT allows globals to be accessed via dedicated capabilities, each with their own permissions and bounds, but comes at the cost of indirection. However, many of those accesses don’t involve direct use of pointers driven by programmer code — this typically occurs only when a pointer is taken to a global, or arbitrary array accesses occur. It may be possible to safely restore use of read-only PC-relative access in many cases, avoiding this indirection.

  • Continuing improvements to the enhanced on-FPGA Morello implementation, such as investigating (and adjusting) additional microarchitectural data structures that were left unchanged from the Neoverse N1 design.

  • Developing and publicizing a model based on performance counters for estimating performance overhead contributions from various essential and non-essential factors on Morello. This may make it easier to estimate potential performance behavior for future microarchitectures using measurements taken on the current microarchitecture.

  • Measuring and understanding the overhead from revocation-based heap temporal safety, part of the CheriBSD 23.11 release, library-based compartmentalisation, available as an experimental opt-in feature since CheriBSD 23.11, and coprocess-based compartmentalisation.

As this work proceeds, we will continue to update this living document.