From: Peter Krempa Date: Fri, 13 May 2022 11:13:02 +0000 (+0200) Subject: qemu: EVENTHANDLERS.txt: Move to kbase and rSTisze X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=e04acdf39d4fdb8c3d28758609a724fa15345f28;p=libvirt.git qemu: EVENTHANDLERS.txt: Move to kbase and rSTisze Signed-off-by: Peter Krempa Reviewed-by: Ján Tomko --- diff --git a/docs/kbase/index.rst b/docs/kbase/index.rst index d0f2167be8..8b710db85a 100644 --- a/docs/kbase/index.rst +++ b/docs/kbase/index.rst @@ -107,3 +107,6 @@ Internals `QEMU migration internals `__ Description of migration phases in the ``v2`` and ``v3`` migration protocol. + +`QEMU monitor event handling `__ + Brief outline how events emitted by qemu on the monitor are handlded. diff --git a/docs/kbase/internals/meson.build b/docs/kbase/internals/meson.build index 4f7b223786..a16d5a290b 100644 --- a/docs/kbase/internals/meson.build +++ b/docs/kbase/internals/meson.build @@ -5,6 +5,7 @@ docs_kbase_internals_files = [ 'locking', 'migration', 'overview', + 'qemu-event-handlers', 'qemu-migration', 'qemu-threads', 'rpc', diff --git a/docs/kbase/internals/qemu-event-handlers.rst b/docs/kbase/internals/qemu-event-handlers.rst new file mode 100644 index 0000000000..3589c4c48c --- /dev/null +++ b/docs/kbase/internals/qemu-event-handlers.rst @@ -0,0 +1,80 @@ +=================== +QEMU event handlers +=================== + +This is a short description of how an example qemu event can be used +to trigger handler code that is called from the context of a worker +thread, rather than directly from the event thread (which should +itself never block, and can't do things like send qemu monitor +commands, etc). + +In this case (the ``NIC_RX_FILTER_CHANGED`` event) the event is handled by +calling a qemu monitor command to get the current RX filter state, +then executing ioctls/sending netlink messages on the host in response +to changes in that filter state. This event is *not* propagated to the +libvirt API (but if someone wants to add details of how to handle that +to the end of this document, please do!). + +Hopefully this narration will be helpful when adding handlers for +other qemu events in the future. + +QEMU monitor events +------------------- + +Any event emitted by qemu is received by +``qemu_monitor_json.c:qemuMonitorJSONIOProcessEvent()``. It looks up the +event by name in the table ``eventHandlers`` (in the same file), which +should have an entry like this for each event that libvirt +understands:: + + { "NIC_RX_FILTER_CHANGED", qemuMonitorJSONHandleNicRxFilterChanged, }, + +NB: This table is searched with bsearch, so it *must* be alphabetically sorted. + +``qemuMonitorJSONIOProcessEvent`` calls the function listed in +``eventHandlers``, e.g.:: + + qemu_monitor_json.c:qemuMonitorJSONHandleNicRxFilterChanged() + +which extracts any required data from the JSON ("name" in this case), +and calls:: + + qemu_monitor.c:qemuMonitorEmitNicRxFilterChanged() + +which uses ``QEMU_MONITOR_CALLBACK()`` to call +``mon->cb->domainNicRxFilterChanged()``. ``domainNicRxFilterChanged`` is one +in a list of function pointers in ``qemu_process.c:monitorCallbacks``. For +our example, it has been set to:: + + qemuProcessHandleNicRxFilterChanged() + +This function allocates a ``qemuProcessEvent`` object, and queues an event +named ``QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED`` (you'll want to add an +enum to ``qemu_domain.h:qemuProcessEventType`` for your event) for a +worker thread to handle. + +(Everything up to this point has happened in the context of the thread +that is reading events from qemu, so it should do as little as +possible, never block, and never call back into the qemu +monitor. Everything after this is handled in the context of a worker +thread, so it has more freedom to make qemu monitor calls and blocking +system calls on the host.) + +When the worker thread gets the event, it calls:: + + qemuProcessEventHandler() + +which switches on the eventType (in our example, +``QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED``) and decides to call:: + + processNicRxFilterChangedEvent() + +and *that* is where the actual work will be done (and any +event-specific memory allocated during ``qemuProcessHandleXXX()`` will be +freed). Note that this function must do proper refcounting of the +domain object, and assure that the domain is still active prior to +performing any operations - it is possible that the domain could have +been destroyed between the time the event was received and the time +that it is processed, and it is also possible that the domain could be +destroyed *during* the event processing if it doesn't get properly +referenced by the handler. diff --git a/src/qemu/EVENTHANDLERS.txt b/src/qemu/EVENTHANDLERS.txt deleted file mode 100644 index 39094d793e..0000000000 --- a/src/qemu/EVENTHANDLERS.txt +++ /dev/null @@ -1,76 +0,0 @@ -This is a short description of how an example qemu event can be used -to trigger handler code that is called from the context of a worker -thread, rather than directly from the event thread (which should -itself never block, and can't do things like send qemu monitor -commands, etc). - -In this case (the NIC_RX_FILTER_CHANGED event) the event is handled by -calling a qemu monitor command to get the current RX filter state, -then executing ioctls/sending netlink messages on the host in response -to changes in that filter state. This event is *not* propagated to the -libvirt API (but if someone wants to add details of how to handle that -to the end of this document, please do!). - -Hopefully this narration will be helpful when adding handlers for -other qemu events in the future. - ----------------------------------------------------- - -Any event emitted by qemu is received by -qemu_monitor_json.c:qemuMonitorJSONIOProcessEvent(). It looks up the -event by name in the table eventHandlers (in the same file), which -should have an entry like this for each event that libvirt -understands: - - { "NIC_RX_FILTER_CHANGED", qemuMonitorJSONHandleNicRxFilterChanged, }, - - NB: This table is searched with bsearch, so it *must* be - alphabetically sorted. - -qemuMonitorJSONIOProcessEvent calls the function listed in -eventHandlers, e.g.: - - qemu_monitor_json.c:qemuMonitorJSONHandleNicRxFilterChanged() - -which extracts any required data from the JSON ("name" in this case), -and calls: - - qemu_monitor.c:qemuMonitorEmitNicRxFilterChanged() - -which uses QEMU_MONITOR_CALLBACK() to call -mon->cb->domainNicRxFilterChanged(). domainNicRxFilterChanged is one -in a list of function pointers in qemu_process.c:monitorCallbacks. For -our example, it has been set to: - - qemuProcessHandleNicRxFilterChanged() - -This function allocates a qemuProcessEvent object, and queues an event -named QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED (you'll want to add an -enum to qemu_domain.h:qemuProcessEventType for your event) for a -worker thread to handle. - -(Everything up to this point has happened in the context of the thread -that is reading events from qemu, so it should do as little as -possible, never block, and never call back into the qemu -monitor. Everything after this is handled in the context of a worker -thread, so it has more freedom to make qemu monitor calls and blocking -system calls on the host.) - -When the worker thread gets the event, it calls - - qemuProcessEventHandler() - -which switches on the eventType (in our example, -QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED) and decides to call: - - processNicRxFilterChangedEvent() - -and *that* is where the actual work will be done (and any -event-specific memory allocated during qemuProcessHandleXXX() will be -freed). Note that this function must do proper refcounting of the -domain object, and assure that the domain is still active prior to -performing any operations - it is possible that the domain could have -been destroyed between the time the event was received and the time -that it is processed, and it is also possible that the domain could be -destroyed *during* the event processing if it doesn't get properly -referenced by the handler.