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Trap Entry/Exit, Interrupt Priority, and Exception Precision: Debug Playbook

Debug Playbook for Trap Entry/Exit, Interrupt Priority, and Exception Precision.

Debug playbook

Debug Playbook for Trap Entry/Exit, Interrupt Priority, and Exception Precision is anchored on Worst-case trap-entry cycles, interrupt response jitter, and precise-exception replay success rate under pipeline backpressure.. 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 - Trap Entry/Exit, Interrupt Priority, and Exception Precision

Worst-case trap-entry cycles, interrupt response jitter, and precise-exception replay success rate under pipeline backpressure. 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 / Trap Entry/Exit, Interrupt Priority, and Exception Precision

1. Symptom
   - Failing metric: Worst-case trap-entry cycles, interrupt response jitter, and precise-exception replay success rate under pipeline backpressure.
   - Failing workload slice: <name>
   - Triggering mode: <privilege/virtualization/interrupt/cache/boot>
   - Build and runtime tags: <toolchain/firmware/RTL revision>

2. Mechanism hypothesis
   - Primary mechanism: RISC-V unifies interrupts and synchronous exceptions through a trap mechanism that records cause and fault context in mode-specific CSRs before redirecting control to a trap vector. On entry, the core snapshots pc into xepc, encodes reason in xcause, and updates xstatus privilege bits; vectoring behavior depends on xtvec mode and cause class. Correctness requires precise exceptions: younger instructions must not commit when an older fault is taken, even with out-of-order retirement and speculative execution. Systems software then uses xret instructions to restore prior privilege and interrupt-enable state, so implementation bugs in save/restore ordering or delegation filters can produce lost interrupts, nested-trap corruption, or privilege escalation paths.
   - 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: Trap sequencing timeline with CSR writes, pipeline flush points, and nested interrupt masking policy.
   - Required owners: CPU microarchitecture lead, interrupt controller architect, firmware runtime owner, kernel exception owner, post-silicon debug owner
   - Final decision: ship, bounded rollout, rollback, or escalate
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