processmission/oh-my-qemu/qemu-tcg-frontend

QEMU TCG Frontend

Audit workflow

For a non-trivial task that writes to the workspace, use only the needed entries under .oh-my-qemu/<task-slug>/: audit.md (baseline, plan, decisions, evidence, verification, and gaps), commands.md (redacted commands, working directories, and results), logs/ (decisive build, test, and runtime output), scripts/ (probes, parsers, and harnesses), and output/ (generated deliverables, third-party dependencies, and non-QEMU binaries). Keep QEMU's own build tree at source-root builds/build-<target>/, never an unqualified build/. In a Git worktree, add .agents/, .oh-my-qemu/, and builds/ to the file from git rev-parse --git-path info/exclude before writing artifacts or configuring QEMU; preserve existing entries and avoid duplicates. Never stage or commit these paths; stage source only when the user explicitly requests that separate action. At handoff, reconcile git status --short with the baseline and report the task directory, build directory, source changes, pre-existing changes, and unresolved gaps.

Source comment hygiene

Keep file-header comments minimal: SPDX/copyright plus at most a one-line component identity. Do not put implementation strategy, framework choice/defense, "modeled contract" pointers, neighboring-device convention, or review/audit rationale in source. Comment only key ISA/decode/backend code, in one to three lines with at most one brief public QEMU or ISA reference; do not expand it into a design or translation essay. Never write .oh-my-qemu/..., output/..., audit.md, evidence paths, round/milestone/AC status, review notes, commands, progress, or absolute host paths into source. When qemu-workflow is installed, run its source-comments gate on changed source files.

QEMU upstream boundary

QEMU's official GitLab and mailing lists are upstream channels. A patch becomes an upstream contribution when it is sent to the maintainers selected through MAINTAINERS. Do not prepare or send agent-generated code, documentation, commit messages, or patches for upstream submission; see the checked-out tree's docs/devel/code-provenance.rst. Research, debugging, analysis, local experiments, verification, and workflow guidance remain allowed. Local branches, commits, patch files, pushes, and pull requests are not by themselves upstream contributions; perform those Git actions only when requested.

Workflow

1. Record the ISA version, encoding, feature/privilege gates, exception behavior, PC/TB rules,

and allowed source paths.

1. Inspect the checked-out decoder and the nearest comparable target before choosing decodetree,

direct TCG ops, or a helper.

1. Work in small reviewable rounds; record changed paths, verification, review result, and gaps.
2. Build the target and run focused valid, invalid, and disabled-feature instruction tests.
3. Capture bounded TCG logs only when needed; distinguish translated, helper, and fallback paths.

Frontend contract

Record in audit.md: ISA version and privilege requirements; instruction encoding, aliases, and operand sign/zero extension; XLEN/vector/FPU constraints; expected illegal-instruction cases; the helper-versus-direct-op decision; and the tests that prove semantics and invalid encodings. Use the ISA coverage template for per-instruction coverage.

Decoder rules

For decodetree targets, reuse existing fields, formats, and argument sets; confirm fixedmask/fixedbits uniqueness; handle overlap groups intentionally; use field functions for transformed immediates; and keep ordering consistent with nearby extensions. For hand decoders, preserve existing fallback and illegal-instruction behavior.

Decoder collision and priority debugging

When an instruction decodes to the wrong pattern or a pattern is unreachable:

1. List the patterns that match the same instruction word and compare fixedbits/fixedmask.
2. Rebuild and read the decodetree generator error or generated decoder path reported by Meson;

conflicts fail generation or leave a later pattern unreachable.

1. Order patterns from most specific to most general; comment intentional overlap groups and add

a negative test for every shadowed encoding.

1. Record the pattern order and tested encodings in audit.md.

trans_* rules

Before emitting IR, gate:

• CPU/ISA feature;
• privilege/virtualization mode;
• XLEN/operand width;
• reserved bits and invalid immediates;
• vector/FPU state where relevant;
• alignment or memory-mode constraints if architectural.

Use direct TCG ops for simple integer/logical/shift/select operations. Use helpers for complex state, softfloat/crypto/vector libraries, or exception-heavy semantics.

Helper side-effect flags

Helper flags tell TCG which globals a helper reads or writes and whether the call has side effects; a helper that raises an exception or mutates guest-visible state must keep those effects visible. Failure example: a raising helper declared with the no-side-effects flag looks removable to TCG, so the call can be dropped or reordered when its result is unused. The build still succeeds; only a guest test that depends on the exception or state write fails. Inspect the checked-out flag definitions (in include/tcg/tcg.h) and add such a test.

PC and TB rules

Check target conventions for:

• pc_next advancement;
• PC update before exceptions;
• branch/direct-block chaining;
• ctx->base.is_jmp state;
• page-boundary behavior;
• single-step and interrupt trigger behavior.

Wrong PC state corrupts exceptions, gdbstub state, and replay/debug evidence.

Custom ISA modeling

For a new toy, vendor, or reverse-engineered ISA target, use custom-isa.md: target enablement, decode and translation registration, TCG op mapping, CPU state and control-register helpers, per-instruction functional tests, qtest coverage, and build targets. Keep the frontend testable per instruction group.

Verification expectations

• Build the target translator.
• Add focused tests/tcg/<target>/ coverage when applicable; otherwise add a functional

per-instruction test.

• Cover edge values, invalid encodings, feature-disabled behavior, and privilege errors.
• Use -accel tcg,one-insn-per-tb=on when debugging instruction boundaries.
• Store TCG logs under .oh-my-qemu/<task-slug>/logs/.

Upstream references

• docs/devel/code-provenance.rst
• docs/devel/decodetree.rst
• docs/devel/tcg.rst
• docs/devel/tcg-ops.rst
• docs/devel/submitting-a-patch.rst
• target/riscv/ (decodetree and insn_trans/ implementation examples)

Paths can differ between QEMU versions; locate them in the checked-out tree.