Re: [Qemu-devel] [RFC 0/9] QContext: QOM class to support multiple event loops

[mdroth]

On Mon, May 06, 2013 at 11:26:06AM +0800, liu ping fan wrote:
> On Sat, May 4, 2013 at 12:03 AM, Michael Roth <address@hidden> wrote:
> > These patches apply on top of qemu.git master, and can also be obtained 
> > from:
> > git://github.com/mdroth/qemu.git qcontext
> >
> > OVERVIEW
> >
> > This series introduces a set of QOM classes/interfaces for event
> > registration/handling: QContext and QSource, which are based closely on
> > their GMainContext/GSource GLib counterparts.
> >
> > QContexts can be created via the command-line via -object, and can also be
> > intructed (via -object params/properties) to automatically start a
> > thread/event-loop to handle QSources we attach to them.
> >
> > The reference implementation of QContext is GlibQContext, which uses
> > GMainContext/GSource interfaces underneath the covers, thus we can
> > also attach GSource (and as a result, AioContexts) to it.
> >
> > As part of this series we also port the QEMU main loop to using QContext
> > and drop virtio-blk's dataplane thread in favor of a GlibQContext thread,
> > which virtio-blk dataplane attaches to via it's AioContext's GSource.
> >
> > With these patches in place we can do virtio-blk dataplane assignment
> > like so:
> >
> >   qemu ... \
> >     -object glib-qcontext,id=ctx0,threaded=yes
> >     -drive file=file.raw,id=drive0,aio=native,cache=none \
> >     -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=ctx0
> >
> > And also gain the ability to assign a virtio-blk dataplane to the default
> > QContext driven by the QEMU main iothread:
> >
> >   qemu ... \
> >     -drive file=file.raw,id=drive0,aio=native,cache=none \
> >     -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=main
> >
> > The latter likely isn't particularly desirable, and the series is in rough
> > shape in general, but the goal of this RFC to demonstrate the approach and
> > get some feedback on how we might handle thread assignments for things like
> > virtio-blk/virtio-net dataplane, and multi-threaded device models general.
> >
> > Input on this would be greatly appreciated.
> >
> > BACKGROUND
> >
> > There has been an outgoing discussion on qemu-devel about what event loop
> > interface to consolidate around for virtio-blk dataplane, threaded 
> > virtio-net,
> > and offloading device workloads to seperate threads in general.
> >
> > Currently the main candidates seem to be GLib GSources and AioContext, with
> > virtio-blk/virtio-net dataplane being the use-cases under consideration.
> >
> > virtio-blk:
> >
> > In the case of virtio-blk dataplane, we need to drive virtqueue and AIO 
> > events.
> > Since AioContext is used extensively throughout the block layer to drive 
> > AIO,
> > it makes sense to re-use it here as we look toward pushing dataplane
> > functionality deeper into the block layer to benefit from things like image
> > format support/snapshots/etc.
> >
> > virtio-net:
> >
> > In the case of Ping Fan's virtio-net dataplane patches
> > (http://thread.gmane.org/gmane.comp.emulators.qemu/196111/focus=196111), we
> > need to drive virtqueue and NetClient peer events (such as TAP devices, or
> > possibly things like slirp in the future). Currently NetClient events rely 
> > on
> > IOHandler interfaces such as qemu_set_fd_handler(). These interfaces are 
> > global
> > ones that rely on a single IOHandler list serviced by QEMU's main loop. An
> > effort is currently underway to port these to GSources so that can be more
> > easilly attached to other event loops (as opposed to the hooks used for the
> > virtio-net dataplane series).
> >
> > Theoretically, much of the latter (such as TAP devices) can also be done 
> > around
> > AioContext with some minor changes, but other NetClient backends such as 
> > slirp
> > lend themselves to more open-ended event loop interfaces like GSources. 
> > Other
> > devices might also find themselves needing something more open-ended (a 
> > somewhat
> > silly but present example being virtio-serial + GSource-driven chardev)
> >
> > QContext:
> >
> > Since the direction for the forseeable future will likely continue to be
> > GSources for some things, AioContext for others, a way to reconcile these
> > approaches would be the age-old approach of adding a layer of abstration on
> > top of the 2 so that we can handle device/backend thread assignments using
> > a general mechanism. Building around this abstration also leaves open the
> > ability to deal with things like locking considerations for real-time 
> > support
> > in the future.
> >
> > A reasonable start to modeling abstraction layer this would be the 
> > open-ended
> > GMainContext/GSource approach that QEMU relies on already, which is what
> > the QContext/QSource interfaces do (with some minor differences/additions
> > such as QSources storing and opaque instead of the GSource-subclassing 
> > approach
> > for GLib).
> >
> I think, custom-ed function for readable or not , ex, tap_can_send()
> should be passed into QSource, but I failed to find such things in
> [PATCH 3/9] QSource: QEMU event source object.  Do I miss it?

We can actually do that via the prepare function. It gets called prior
to polling the GPollFDs for that QSource/GSource. In the case where
tap_can_send() == false, we'd simply zero out the GPollFD.events for the
tap device in prepare().

One other interesting option this series opens up with regard to
NetClient that currently use qemu_set_fd_handler():

We now have set_fd_handler(ctx, ...) interfaces, which assigns FD handlers
to a specific QContext/event loop. So we don't actually need to convert all
NetClients to GSources to offload handling to a dataplane thread. I
think we'd still want to use a GSource for slirp though.

Not saying that approach is better though, but maybe something to
consider.

> 
> Thanks and regards,
> Pingfan
> > TODO/KNOWN ISSUES
> >
> >  - GTK UI causes a crash during certain window refresh events. works fine 
> > with
> >    VNC though, and no problems with other GSources.
> >  - slirp isn't working (will work with Ping Fan's slirp->GSource conversion)
> >  - add proper locking
> >  - integrate timers into a QSource to make timer-event driveable in seperate
> >    QContext event loops
> >  - consider a command-line wrapper to -object, such as:
> >      -context <id>,[threaded=<yes|no>],[backend=<default|glib>]
> >
> >  Makefile.objs                    |    6 +-
> >  async.c                          |   16 +++
> >  hw/block/dataplane/virtio-blk.c  |   46 ++-----
> >  include/block/aio.h              |    6 +
> >  include/hw/virtio/virtio-blk.h   |    7 +-
> >  include/qcontext/glib-qcontext.h |   59 ++++++++
> >  include/qcontext/qcontext.h      |   76 +++++++++++
> >  include/qcontext/qsource.h       |   63 +++++++++
> >  include/qemu/main-loop.h         |   31 ++++-
> >  include/qom/object.h             |   35 +++++
> >  iohandler.c                      |  238 ++++++++++++++++++++++----------
> >  main-loop.c                      |   96 ++++++-------
> >  qcontext/Makefile.objs           |    1 +
> >  qcontext/glib-qcontext.c         |  280 
> > ++++++++++++++++++++++++++++++++++++++
> >  qcontext/qcontext.c              |  252 ++++++++++++++++++++++++++++++++++
> >  qcontext/qsource.c               |  143 +++++++++++++++++++
> >  qom/Makefile.objs                |    6 +-
> >  qom/object.c                     |   46 +++++++
> >  tests/Makefile                   |    7 +
> >  tests/test-qcontext.c            |  259 +++++++++++++++++++++++++++++++++++
> >  vl.c                             |    2 +
> >  21 files changed, 1512 insertions(+), 163 deletions(-)
> >
>