Features/CoroutineFnCleanup

Hand-written wrappers for coroutine functions

1. bdrv_pread/bdrv_pwrite have different calling convention than bdrv_co_pread/bdrv_co_pwrite [Alberto]
   • Fix using Coccinelle
   • Switch bdrv_pread/bdrv_pwrite to generated_co_wrapper
2. Missing bdrv_co_pwrite_sync, bdrv_co_pwrite_zeroes
   • Adjust code in block/io.c and introduce generated_co_wrapper
3. Use generated_co_wrapper for block-backend.c too, removing the need for declarations in block/coroutines.h [Alberto]
4. Probably more will be found with custom matching tools (see below)

Converting callbacks to coroutines

Conversion of GLOBAL_STATE_CODE() callbacks to coroutines depends on removal of the AioContext lock:

• from coroutine context, IO_OR_GS_CODE() functions such as "drain_begin" have to be called with the AioContext lock taken
• to call them with the lock taken, the code has to run in the BDS's home iothread...
• ... and that is not possible for GLOBAL_STATE_CODE() functions unless the BDS home iothread is the main thread (which cannot be guaranteed)

IO_CODE() callbacks can be converted to coroutines. Even for functions may be GLOBAL_STATE_CODE(), the underlying callbacks can be an IO_CODE() coroutine_fn. For example:

• bdrv_inactivate, which does I/O, can be converted to a coroutine_fn callback. It is only implemented by qcow2, so it can be called from bdrv_close instead of qcow2_close. The call in bdrv_inactivate_recurse would go through a generated_co_wrapper in block/coroutines.h, e.g. bdrv_do_inactivate_one.
• It should be possible to convert the bdrv_snapshot_goto callback too. Likewise, bdrv_snapshot_goto would call a generated_co_wrapper, e.g. bdrv_do_snapshot_goto.

Annotations

1. Add coroutine_mixed_fn annotation; functions that call qemu_in_coroutine(). Leaves are:
   • all generated_co_wrapper functions
   • bdrv_create
   • bdrv_can_store_new_dirty_bitmap, bdrv_remove_persistent_dirty_bitmap
   • bdrv_drain_all_begin, bdrv_do_drained_begin, bdrv_do_drained_end
   • qio_channel_readv_full_all_eof, qio_channel_writev_full
   • qcow2_open, qcow2_has_zero_init
   • bdrv_qed_open
   • qmp_dispatch
   • nvme_get_free_req
   • schedule_next_request
2. Detect call from non-coroutine_fn/coroutine_mixed_fn to coroutine_fn (libclang/clang-tidy)
   • Remove coroutine_fn from callee, or add it to caller (check against https://patchew.org/QEMU/20220509103019.215041-1-pbonzini@redhat.com/)
   • Possibly add fixit to rewrite
3. Detect call from coroutine_fn to generated_co_wrapper (libclang/clang-tidy)
   • Use _co_ version instead.
   • Possibly add fixit to rewrite
4. Detect call from non-coroutine_fn to coroutine_mixed_fn (libclang/clang-tidy)
   • Change caller to coroutine_mixed_fn too
   • If there is none, coroutine_mixed_fn can be changed to coroutine_fn

Clang-query examples

Invoking clang-query:

1. Change definition of coroutine_fn to #define coroutine_fn __attribute__((annotate("coroutine")))
2. Invoke clang-query -p build-path block/io.c

Example of clang-query uses:

match functionDecl(allOf(unless(hasAttr("attr::Annotate")),hasDescendant(callExpr(callee(hasAttr("attr::Annotate"))).bind("callee"))))

call from non-coroutine_fn to coroutine_fn (doesn't actually check the string in the annotate attribute)

match varDecl(hasType(namedDecl(hasName("BlockDriver"))), hasInitializer(anything()))

declaration of BlockDriver struct, with initializer

match functionDecl(isExpansionInMainFile(), forEachDescendant(callExpr(callee(unless(functionDecl()))).bind("call"))).bind("root")'

indirect call

Clang references

https://devblogs.microsoft.com/cppblog/exploring-clang-tooling-part-1-extending-clang-tidy/

https://devblogs.microsoft.com/cppblog/exploring-clang-tooling-part-2-examining-the-clang-ast-with-clang-query/

https://devblogs.microsoft.com/cppblog/exploring-clang-tooling-part-3-rewriting-code-with-clang-tidy/

VRC / path search

VRC is a tool to query a call graph. The call graph can be obtained from either GCC debug dumps (-fdump-rtl-expand) or clang (experimental).

The query language is based on regular expressions:

[label1]

Find function with the specified __attribute((annotate)) annotation

funcname, /regex/

Find functions with the given name or matching the regex.

...

Zero or more functions (same as .* in grep, see below)

( ... | ... )

Alternation

Anything except ... can be followed by a star to mean 0 or more occurrences.

[label1] is actually its own mini-language to query individual nodes:

[label1,label2]

Find function with all the specified __attribute((annotate)) annotations

[] matches all functions, i.e. ... is a clearer alternative to []*

![label1|label2]

Find function with none of the specified __attribute((annotate)) annotations

["func1"], [/regex/]

Match on function name, for example [coroutine_fn, !"qemu_coroutine_yield"].

It is also possible to walk through edges; for example, coroutine candidates can be labeled with a single command: label coroutine_fn_candidate [!coroutine_fn,!coroutine_mixed_fn,coroutine_fn:callees,![!coroutine_fn:callees]] i.e. find the functions that are

• not coroutine_fn or coroutine_mixed_fn
• called by coroutine_fn
• not called by non-coroutine_fn

While the query language currently can only be used from the command line, it is possible to construct the AST from a Python program and therefore build static analysis that can be run at "make check" time.

Sample call path queries

paths ![coroutine_fn|coroutine_mixed_fn] [coroutine_fn]

Find incorrect calls to coroutine_fn

paths [coroutine_fn] [no_coroutine_fn]

Find incorrect calls to no_coroutine_fn

paths [coroutine_fn] ![coroutine_fn|coroutine_mixed_fn]

paths ![coroutine_fn] ![coroutine_fn|coroutine_mixed_fn]

Find functions called only from functions that are coroutine_fn, and respectively not coroutine_fn. Functions only found in the first set are candidates for labeling as coroutine_fn, as in the "label" command above.

label coroutine_fn_candidate [!coroutine_fn,!coroutine_mixed_fn,coroutine_fn:callees,![!coroutine_fn:callees]]
paths [coroutine_fn_candidate] ... qemu_coroutine_yield

Find coroutine_fn_candidates (see above) that ultimately suspend. These are slam-dunk candidates for labeling as coroutine_fn.

label suspending_fn ["qemu_coroutine_yield":all_callers]
paths [coroutine_fn_candidate,suspending_fn]

More efficient version of the above

paths [coroutine_mixed_fn,!coroutine_fn:callers]

Find mixed functions called from functions that are not coroutine_fn. Any coroutine_mixed_fn functions not on the list are candidates for changing to coroutine_fn

paths [coroutine_mixed_fn,![coroutine_mixed_fn,!coroutine_fn:callers]]

Find the "bad" coroutine_mixed_fn as mentioned in the previous query.

label suspending_fn ["qemu_coroutine_yield":all_callers]
label no_coroutine_candidates [coroutine_mixed_fn,![suspending_fn:callers]]
paths [coroutine_fn|[coroutine_mixed_fn,!no_coroutine_candidates]] [no_coroutine_candidates]

Find coroutine_mixed_fn that will never suspend the current coroutine. These are candidates for switching to no_coroutine_fn.