Markus Armbruster <arm...@redhat.com> writes: > Kevin Wolf <kw...@redhat.com> writes: > >> Am 15.01.2020 um 15:59 hat Markus Armbruster geschrieben: >>> Kevin Wolf <kw...@redhat.com> writes: >>> >>> > This patch adds a new 'coroutine' flag to QMP command definitions that >>> > tells the QMP dispatcher that the command handler is safe to be run in a >>> > coroutine. >>> > >>> > Signed-off-by: Kevin Wolf <kw...@redhat.com> >>> > Reviewed-by: Marc-André Lureau <marcandre.lur...@redhat.com> >>> > --- >>> > tests/qapi-schema/qapi-schema-test.json | 1 + >>> > docs/devel/qapi-code-gen.txt | 4 ++++ >>> > include/qapi/qmp/dispatch.h | 1 + >>> > tests/test-qmp-cmds.c | 4 ++++ >>> > scripts/qapi/commands.py | 17 +++++++++++------ >>> > scripts/qapi/doc.py | 2 +- >>> > scripts/qapi/expr.py | 4 ++-- >>> > scripts/qapi/introspect.py | 2 +- >>> > scripts/qapi/schema.py | 9 ++++++--- >>> > tests/qapi-schema/qapi-schema-test.out | 2 ++ >>> > tests/qapi-schema/test-qapi.py | 7 ++++--- >>> > 11 files changed, 37 insertions(+), 16 deletions(-) >>> > >>> > diff --git a/tests/qapi-schema/qapi-schema-test.json >>> > b/tests/qapi-schema/qapi-schema-test.json >>> > index 9abf175fe0..1a850fe171 100644 >>> > --- a/tests/qapi-schema/qapi-schema-test.json >>> > +++ b/tests/qapi-schema/qapi-schema-test.json >>> > @@ -147,6 +147,7 @@ >>> > 'returns': 'UserDefTwo' } >>> > >>> > { 'command': 'cmd-success-response', 'data': {}, 'success-response': >>> > false } >>> > +{ 'command': 'coroutine-cmd', 'data': {}, 'coroutine': true } >>> > >>> > # Returning a non-dictionary requires a name from the whitelist >>> > { 'command': 'guest-get-time', 'data': {'a': 'int', '*b': 'int' }, >>> > diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt >>> > index 45c93a43cc..753f6711d3 100644 >>> > --- a/docs/devel/qapi-code-gen.txt >>> > +++ b/docs/devel/qapi-code-gen.txt >>> > @@ -457,6 +457,7 @@ Syntax: >>> > '*gen': false, >>> > '*allow-oob': true, >>> > '*allow-preconfig': true, >>> > + '*coroutine': true, >>> > '*if': COND, >>> > '*features': FEATURES } >>> > >>> > @@ -581,6 +582,9 @@ before the machine is built. It defaults to false. >>> > For example: >>> > QMP is available before the machine is built only when QEMU was >>> > started with --preconfig. >>> > >>> > +Member 'coroutine' tells the QMP dispatcher whether the command handler >>> > +is safe to be run in a coroutine. It defaults to false. >>> >>> Two spaces after sentence-ending period, for consistency with the rest >>> of the file. >> >> Ok. >> >>> As discussed in review of prior versions, coroutine-safety is an >>> implementation detail that should not be exposed to management >>> applications. Therefore, we want a flag, not a feature. >>> >>> As far as I can tell, the new flag has no effect until PATCH 3 puts it >>> to use. That's okay. >>> >>> The doc update tells us when we may say 'coroutine': true, namely when >>> the handler function is coroutine-safe. It doesn't quite tell us what >>> difference it makes, or rather will make after PATCH 3. I think it >>> should. >> >> Fair requirement. Can I describe it as if patch 3 were already in? That >> is, the documentation says that the handler _will_ be run in a coroutine >> rather than _may_ run it in a coroutine? > > Your choice. If you choose to pretend PATCH 3 was in, have your commit > message point that out. > >>> In review of a prior version, Marc-André wondered whether keeping >>> allow-oob and coroutine separate makes sense. Recall qapi-code-gen.txt: >>> >>> An OOB-capable command handler must satisfy the following conditions: >>> >>> - It terminates quickly. >>> - It does not invoke system calls that may block. >>> - It does not access guest RAM that may block when userfaultfd is >>> enabled for postcopy live migration. >>> - It takes only "fast" locks, i.e. all critical sections protected by >>> any lock it takes also satisfy the conditions for OOB command >>> handler code. >>> >>> The restrictions on locking limit access to shared state. Such access >>> requires synchronization, but OOB commands can't take the BQL or any >>> other "slow" lock. >>> >>> Kevin, does this rule out coroutine use? >> >> Not strictly, though I also can't think of a case where you would want >> to use a coroutine with these requirements. >> >> If I understand correctly, OOB-capable commands can be run either OOB >> with 'exec-oob' or like normal commands with 'execute'. > > Correct. > >> If an OOB >> handler is marked as coroutine-safe, 'execute' will run it in a >> coroutine (and the restriction above don't apply) and 'exec-oob' will >> run it outside of coroutine context. > > Let me convince myself you're right. > > Cases before this series: > > (exec) execute, allow-oob does not matter > > Run in main loop bottom half monitor_qmp_bh_dispatcher(), outside > coroutine context, scheduled by handle_qmp_command() > > (err1) exec-oob, QMP_CAPABILITY_OOB off, allow-oob does not matter > > Error > > (err2) exec-oob, QMP_CAPABILITY_OOB on, allow-oob: false > > Error > > (exec-oob) exec-oob, QMP_CAPABILITY_OOB on, allow-oob: true > > Run in iothread / handle_qmp_command(), outside coroutine context > > Peeking ahead to PATCH 3... it split cases (exec): > > (exec-co): execute, allow-oob does not matter, coroutine: true > > Run in main loop coroutine qmp_dispatcher_co(), in coroutine context, > woken up by handle_qmp_command() > > (exec): execute, allow-oob does not matter, coroutine: false > > Run in main loop bottom half do_qmp_dispatch_bh(), outside coroutine > context, scheduled by qmp_dispatcher_co() > > It appears not to touch case exec-oob. Thus, coroutine: true has no > effect when the command is executed with exec-oob.
Looking at PATCH 3 again, I got temporarily confused again. Let me spell things out even more, to improve my chances at staying not confused... To effect the split of (exec), you rewrite bottom half monitor_qmp_bh_dispatcher() as coroutine monitor_qmp_dispatcher_co(), then have do_qmp_dispatch() either call the the handler directly, or schedule it to run in a bottom half. Cases: (exec-co): handle_qmp_command() sends the command to coroutine monitor_qmp_dispatcher_co(), which calls monitor_qmp_dispatch(), which runs the handler, in coroutine context, in the main loop. (exec): Likewise, except monitor_qmp_dispatch() schedules the handler to run in a bottom half, outside coroutine context, in the main loop. (exec-oob): handle_qmp_command() runs monitor_qmp_dispatch(), which runs the handler, outside coroutine context, in the iothread. > Looks like you're right :) [...]