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CSR Programming Model, Privilege Visibility, and Timer Facilities: Debug Playbook

Debug Playbook for CSR Programming Model, Privilege Visibility, and Timer Facilities.

Debug playbook

Debug Playbook for CSR Programming Model, Privilege Visibility, and Timer Facilities is anchored on CSR read/write hazard rate, timer interrupt accuracy (ns error), and software context-switch overhead attributable to CSR traffic.. Convert observations into mechanism-backed decisions with explicit ownership.

  1. Freeze workload seed, binary, compiler, firmware, and platform setup.

  2. Find earliest persistent mechanism failure in the timeline.

  3. Build one reduced reproducer for dominant hypothesis.

  4. Patch minimal fix with explicit rollback gate.

  5. Rerun correctness + performance + power matrix.

Decision tree

diagram
ROOT CAUSE TREE - CSR Programming Model, Privilege Visibility, and Timer Facilities

CSR read/write hazard rate, timer interrupt accuracy (ns error), and software context-switch overhead attributable to CSR traffic. regressed
          |
   reproducible on fixed seed?
      /                 \
    no                   yes
    |                     |
env/tool drift       first failing domain?
                     /        |         \
                  decode    execute    memory/MMU
                    |         |            |
               control map  bypass/FU   TLB/walk/perm
                    |
         privilege/CSR side effects checked?

Stop at first confirmed mechanism, then assign explicit owner + fix proof.

Review memo template

diagram
RISC-V REVIEW MEMO - Privileged Architecture / CSR Programming Model, Privilege Visibility, and Timer Facilities

1. Symptom
   - Failing metric: CSR read/write hazard rate, timer interrupt accuracy (ns error), and software context-switch overhead attributable to CSR traffic.
   - Failing workload slice: <name>
   - Triggering mode: <privilege/virtualization/interrupt/cache/boot>
   - Build and runtime tags: <toolchain/firmware/RTL revision>

2. Mechanism hypothesis
   - Primary mechanism: Control and Status Registers (CSRs) expose the architectural control plane for privilege management, interrupt state, memory translation setup, performance counters, and platform extension features. Each CSR has a defined privilege and access type (read-only, WARL, side-effecting), so hardware must enforce legal writes and deterministic read behavior even when software probes optional features. Timekeeping commonly relies on mtime/mtimecmp through machine-level timer infrastructure, with delegation or extension support enabling supervisor-visible timer interrupts for OS scheduling. High-quality designs define clear clock-domain crossing and ordering semantics for timer updates, because race conditions between compare writes and pending interrupts can generate spurious ticks or missed scheduling deadlines.
   - Competing hypotheses: <decode control / hazard / MMU / privilege / integration>
   - Missing evidence: <trace/counter/proof/report>

3. Proposed action
   - Smallest reversible change: <RTL/compiler/runtime/config>
   - Expected movement: <throughput/latency/tail/perf-per-watt>
   - Regression risk: correctness, security, power, timing, software compatibility

4. Signoff
   - Required artifact: CSR ownership map plus timer interrupt programming guide including atomic update sequence and race-avoidance checklist.
   - Required owners: privileged ISA owner, platform timer owner, firmware and SBI owner, kernel scheduler owner, verification owner
   - Final decision: ship, bounded rollout, rollback, or escalate
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