Benchmark ABI

The Benchmark ABI makes minor changes to pure-capability code generation to address a limitation in Morello's branch prediction when PCC bounds may change. This limitation penalizes the performance of function calls and returns where PCC bounds are required shortly after the jump, leading to instruction stalls. Benchmark ABI code uses integer jumps rather than capability jumps, and Benchmark ABI processes use a PCC with bounds covering the full address space. Further details regarding Morello performance, and the Benchmark ABI, may be found in Early performance results from the prototype Morello microarchitecture.

This activity involves source code in the benchmark subdirectory:

cd ~/tutorial/src/benchmark

Compiling code for the Benchmark ABI

Compile helloworld.c as a CheriABI binary:

cc -Wall -g -o helloworld-cheriabi helloworld.c

Compile helloworld.c as a Benchmark ABI binary:

cc -Wall -g -mabi=purecap-benchmark -o helloworld-benchmark helloworld.c

Identifying Benchmark ABI binaries

Use the file(1) command to identify the two binaries:

file helloworld-cheriabi

file helloworld-benchmark

Inspect ELF binary headers with readelf (from the base system) or llvm-readelf (from LLVM for Morello):

readelf -n helloworld-cheriabi

readelf -n helloworld-benchmark

llvm-readelf -n helloworld-cheriabi

llvm-readelf -n helloworld-benchmark

Running Benchmark ABI binaries

Run both binaries from the command line:

./helloworld-cheriabi

./helloworld-benchmark

The Benchmark ABI package manager

Using pkg64cb, which manages a complete set of third-party software packages compiled for the Benchmark ABI, list the currently installed Benchmark ABI package:

pkg64cb info

Now install the Benchmark ABI compilation of bash:

sudo pkg64cb install bash

You can read more on useful package manager commands in the Getting Started with CheriBSD guide.

Identifying Benchmark ABI processes

In one terminal window, run the Benchmark ABI version of the bash shell with

/usr/local64cb/bin/bash

and execute in the bash shell session

echo $$

to print the process's PID. Note that PID down.

In a second terminal window, run

procstat -a

to list the ABIs for all running processes.

• What is shown for your Benchmark ABI bash process with the PID you noted down, and how does this differ from other processes you see?

Disassembling Benchmark ABI binaries

Use objdump to disassemble the main() functions in both binaries:

objdump -dj .text ./helloworld-cheriabi

objdump -dj .text ./helloworld-benchmark

• What differences exist between the two functions, and why?

Debugging Benchmark ABI binaries

Use GDB to analyse CPU register contents just before and after returning from the main() functions in both binaries.

First, run gdb:

gdb ./helloworld-cheriabi

Once you enter a GDB session,

1. Disassemble the main() function:

   disassemble main
   

2. Record the offset of the RET instruction at the end of disassembly, e.g. 56 in

   0x00000000001107ec <+56>:    ret     c30
   

3. Set a breakpoint for the RET instruction:

   break *main + N
   

   where N is the offset you recorded, e.g.

   break *main + 56
   

4. Run the program:

   run
   

5. Disassemble the current function to make sure the process was suspended at the RET instruction:

   disassemble
   

6. Display the C30 and PCC registers:

   info register c30 pcc
   

7. Execute the RET instruction:

   stepi
   

8. Display the PCC register again:

   info register pcc
   

Repeat the GDB session for the Benchmark ABI binary:

gdb ./helloworld-benchmark

• What differences exist between the two functions, and why?

Benchmarking with the Benchmark ABI

Compile the source code for two short C programs, benchmark-atoi.c and benchmark-sha256.c:

cc -Wall -g -o benchmark-atoi-cheriabi benchmark-atoi.c -lmd

cc -Wall -g -mabi=purecap-benchmark -o benchmark-atoi-benchmarkabi benchmark-atoi.c -lmd

cc -Wall -g -o benchmark-sha256-cheriabi benchmark-sha256.c -lmd

cc -Wall -g -mabi=purecap-benchmark -o benchmark-sha256-benchmarkabi benchmark-sha256.c -lmd

Using the UNIX time(1) command, run each of the resulting binaries:

time ./benchmark-atoi-cheriabi

time ./benchmark-atoi-benchmarkabi

time ./benchmark-sha256-cheriabi

time ./benchmark-sha256-benchmarkabi

• How long does execution take for the CheriABI vs Benchmark ABI compilations of each program?
• Review the source code for the two workloads; why do they perform the way that they do?