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Virtual Memory with Sv39/Sv48, TLB Behavior, and Page-Walk Semantics: Debug Playbook

Debug Playbook for Virtual Memory with Sv39/Sv48, TLB Behavior, and Page-Walk Semantics.

Debug playbook

Debug Playbook for Virtual Memory with Sv39/Sv48, TLB Behavior, and Page-Walk Semantics is anchored on TLB miss rate, page-walk latency distribution, and stale-translation incidents after mapping changes.. 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 - Virtual Memory with Sv39/Sv48, TLB Behavior, and Page-Walk Semantics

TLB miss rate, page-walk latency distribution, and stale-translation incidents after mapping changes. 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 / Virtual Memory with Sv39/Sv48, TLB Behavior, and Page-Walk Semantics

1. Symptom
   - Failing metric: TLB miss rate, page-walk latency distribution, and stale-translation incidents after mapping changes.
   - Failing workload slice: <name>
   - Triggering mode: <privilege/virtualization/interrupt/cache/boot>
   - Build and runtime tags: <toolchain/firmware/RTL revision>

2. Mechanism hypothesis
   - Primary mechanism: Sv39 and Sv48 define multi-level page-table translation with 39-bit and 48-bit virtual address schemes, respectively, using satp-selected root pointers and PTE permission bits to map pages at multiple granularities. Hardware page walkers traverse levels, validate access and privilege rules, and populate translation caches; leaf and non-leaf encoding mistakes must raise deterministic faults rather than creating silent aliasing. System correctness depends on shootdown discipline: after software edits page tables, sfence.vma synchronization is required so harts discard stale translations and permission metadata. Performance tradeoffs include page size selection, TLB hierarchy sizing, and address-space identifier usage, all of which influence context-switch cost and memory-isolation robustness.
   - 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: Sv39/Sv48 translation flowchart with PTE validity checks, fault taxonomy, and sfence.vma shootdown protocol.
   - Required owners: MMU architect, kernel memory-management owner, virtualization owner, performance modeling owner, formal verification owner
   - Final decision: ship, bounded rollout, rollback, or escalate
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