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Tockilator: Deducing Tock execution flows from Ibex Verilator traces

Tockilator: Deducing Tock execution flows from Ibex Verilator traces

Talk given on March 20, 2020 at Oxidize 1K, a virtual conference that went from first idea to 300+ person conference in a week during the COVID-19 pandemic. Video: https://www.youtube.com/watch?v=zPuELAzJyno#t=10534s

Bryan Cantrill

March 20, 2020
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  1. Tockilator
    Deducing Tock execution flow
    from Ibex Verilator traces
    Bryan Cantrill
    Oxide Computer Company

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  2. Oxide?
    ● New computer company seeking to modernize on-premises computers
    ● We are rethinking the embedded systems at the foundation of
    computers: the hardware root-of-trust and service processor
    ● With its emphasis on safety, security, and size, Rust is an excellent fit for
    these applications

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  3. Tock
    ● Tock is a Rust-based operating system designed for embedded systems
    ● A logical descendent of TinyOS, retains the emphasis on small size while
    allowing the system to be extensible, portable and reliable
    ● Tock allows for MPU-/PMP-protected applications
    ● Importantly, Tock aims for application portability
    ● As such, promising for collaboration across different embedded
    projects -- and being used for the OpenTitan root-of-trust project

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  4. Verilator + Ibex
    ● Verilator is an open source, cycle-accurate simulation tool that takes
    synthesizable Verilog, and translates it to C++ to execute
    ● Verilator can emit e.g. FST for use as input to GTKwave
    ● Verilator is being used to simulate Ibex, an open 32-bit RISC-V core
    ● Ibex is the softcore for Earl Grey, the first OpenTitan SoC
    ● Ibex has a tracer module that has been modified to support Verilator
    with human-readable instruction logging

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  5. Complete instruction tracing!
    ● Debugging embedded systems is extraordinarily excruciating...
    ● ...but having entire instruction traces is singularly valuable!
    ● Looking at instruction traces can enable bringup -- and obviously has
    tremendous power around code coverage, memory coverage, etc.
    ● Challenge: as tedious as instruction traces are to use to understand
    C-based systems, they are much, much harder for Rust-based systems!
    ● Can we process Ibex Verilator traces to make sense of Tock?

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  6. Tockilator
    ● Rust program that takes an Ibex Verilator trace and a pointer to the ELF
    objects representing the boot ROM and Tock instance
    ● Symbolically resolves instructions; by determining function entry and
    return, can show execution flow through Tock
    ● Tockilator uses DWARF information to decode function parameters
    and (importantly!) inlined function calls
    ● Tock architecture allows Tockilator to show app + kernel flow
    ● Primordial, but promising for Tock and Tock-based applications!

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  7. Rust, Tockilator and beyond
    ● Rust is an excellent fit for embedded systems: its ability to have
    powerful abstractions with tiny binaries borders on the magical
    ● But to debug systems, we must cut through the magic, allowing the
    system to be understood from the machine up
    ● Tockilator is one approach, but there is much more than can be done;
    instruction traces are a very rich vein for system understanding!
    ● Tockilator is available at https://github.com/oxidecomputer/tockilator

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