Re: [Qemu-devel] [RFC 4/6] CLI: add -paused option

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Oct 20, 2017 at 12:19:17PM +1100, David Gibson wrote:
> On Thu, Oct 19, 2017 at 10:15:48PM -0200, Eduardo Habkost wrote:
> > On Thu, Oct 19, 2017 at 09:42:18PM +1100, David Gibson wrote:
> > > On Mon, Oct 16, 2017 at 02:59:16PM -0200, Eduardo Habkost wrote:
> > > > On Mon, Oct 16, 2017 at 06:22:54PM +0200, Igor Mammedov wrote:
> > > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > > > ---
> > > > >  include/sysemu/sysemu.h |  1 +
> > > > >  qemu-options.hx         | 15 ++++++++++++++
> > > > >  qmp.c                   |  5 +++++
> > > > >  vl.c                    | 54 ++++++++++++++++++++++++++++++++++++++++++++++++-
> > > > >  4 files changed, 74 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> > > > > index b213696..3feb94f 100644
> > > > > --- a/include/sysemu/sysemu.h
> > > > > +++ b/include/sysemu/sysemu.h
> > > > > @@ -66,6 +66,7 @@ typedef enum WakeupReason {
> > > > >      QEMU_WAKEUP_REASON_OTHER,
> > > > >  } WakeupReason;
> > > > >  
> > > > > +void qemu_exit_preconfig_request(void);
> > > > >  void qemu_system_reset_request(ShutdownCause reason);
> > > > >  void qemu_system_suspend_request(void);
> > > > >  void qemu_register_suspend_notifier(Notifier *notifier);
> > > > > diff --git a/qemu-options.hx b/qemu-options.hx
> > > > > index 39225ae..bd44db8 100644
> > > > > --- a/qemu-options.hx
> > > > > +++ b/qemu-options.hx
> > > > > @@ -3498,6 +3498,21 @@ STEXI
> > > > >  Run the emulation in single step mode.
> > > > >  ETEXI
> > > > >  
> > > > > +DEF("paused", HAS_ARG, QEMU_OPTION_paused, \
> > > > > +    "-paused [state=]postconf|preconf\n"
> > > > > +    "                postconf: pause QEMU after machine is initialized\n"
> > > > > +    "                preconf: pause QEMU before machine is initialized\n",
> > > > > +    QEMU_ARCH_ALL)
> > > > 
> > > > I would like to allow pausing before machine-type is selected, so
> > > > management could run query-machines before choosing a
> > > > machine-type.  Would that need a third "-pause" mode, or will we
> > > > be able to change "preconf" to pause before select_machine() is
> > > > called?
> > > > 
> > > > The same probably applies to other things initialized before
> > > > machine_run_board_init() that could be configurable using QMP,
> > > > including but not limited to:
> > > > * Accelerator configuration
> > > > * Registering global properties
> > > > * RAM size
> > > > * SMP/CPU configuration
> > > 
> > > Yeah.. having a bunch of different possible pause stages to select
> > > doesn't sound great.
> > 
> > I agree.  The number of externally visible pause states should be
> > as small as possible.
> > 
> > 
> > >                       Could we avoid this by instead changing -S to
> > > pause at the earliest possible spot, but having any monitor commands
> > > that require a later stage automatically "fast forwarding" to the
> > > right phase?
> > 
> > That would hide the internal details from the outside.  Sounds
> > nice, but adding new machine/device configuration QMP commands
> > while hiding the QEMU state from the outside sounds impossible.
> > 
> > For example, if we use -S today, this works:
> > 
> >   $ qemu-system-x86_64 -S -qmp stdio
> >   <- {"QMP": {"version": {"qemu": {"micro": 0, "minor": 10, "major": 2}, "package": " (v2.10.0-83-g9375da7831)"}, "capabilities": []}}
> >   -> {"execute":"qmp_capabilities"}
> >   <- {"return": {}}
> >   -> {"execute":"query-cpus"}
> >   <- {"return": [{"arch": "x86", "current": true, "props": {"core-id": 0, "thread-id": 0, "socket-id": 0}, "CPU": 0, "qom_path": "/machine/unattached/device[0]", "pc": 4294967280, "halted": false, "thread_id": 4038}]}
> > 
> > This means "query-cpus" needs to fast-forward to the CPU creation
> > stage if we want to keep compatibility.
> > 
> > Now, assume we add a set-numa-node command like the one in this
> > series.  e.g.:
> > 
> >   $ qemu-system-x86_64 -S -qmp stdio
> >   <- {"QMP": {"version": {"qemu": {"micro": 0, "minor": 10, "major": 2}, "package": " (v2.10.0-83-g9375da7831)"}, "capabilities": []}}
> >   -> {"execute":"qmp_capabilities"}
> >   <- {"return": {}}
> >   -> {"execute":"set-numa-node" ... }
> >   <- {"return": ...}
> > 
> > The command will work only if machine initialization didn't run
> > yet.
> > 
> > But now an innocent-looking query command would change QEMU state
> > in an unexpected way:
> > 
> >   $ qemu-system-x86_64 -S -qmp stdio
> >   <- {"QMP": {"version": {"qemu": {"micro": 0, "minor": 10, "major": 2}, "package": " (v2.10.0-83-g9375da7831)"}, "capabilities": []}}
> >   -> {"execute":"qmp_capabilities"}
> >   <- {"return": {}}
> >   -> {"execute":"query-cpus"}  [will silently fast-forward QEMU state]
> >   <- {"return": [{"arch": "x86", "current": true, "props": {"core-id": 0, "thread-id": 0, "socket-id": 0}, "CPU": 0, "qom_path": "/machine/unattached/device[0]", "pc": 4294967280, "halted": false, "thread_id": 4038}]}
> >   -> {"execute":"set-numa-node" ... }
> >   <- {"error": ...}  [the command will fail because the machine was already created]
> > 
> > This means we do have a externally visible "too late to use
> > set-numa-node" QEMU state, and query-cpus will have a externally
> > visible side effect.  Every QMP command would need to document
> > how it affects QEMU state in a externally visible way.
> > 
> > If QEMU pause state is still going to be externally visible this
> > way, I would prefer to let the client to explicitly tell what's
> > the state they want QEMU to be, instead of making QEMU change
> > state silently as a side effect of QMP commands.
> 
> Yeah, good point.  My proposal would just have changed explicitly
> exposed ugly internal state to subtly exposed ugly internal state,
> which is probably worse :(.
> 
> 
> Ok.. next possibly bad idea..
> 
> What about a "re-exec" monitor command; it would take what's
> essentially a new command line, and basically restart qemu from the
> beginning, reparsing this new command line, but without actually 
> 
> Pro:
>   * Mitigates Daniel Berrange's concern about lots of qemu
>     configuration being buried in the qmp session - if libvirt logged
>     its last "re-exec" that would have what is generally needed.
>   * Lets libvirt do assorted investigation of options, then rewind to
>     choose what it actually wants

Sounds like a superset of Paolo's "-machine none" proposal[1].
It would be a very simple interface, not sure it can be easily
implemented efficiently.

[1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg488618.html

> 
> Con:
>   * Would require a bunch of auditing of structures/state to make sure
>     they can be re-initialized cleanly

This sounds like a big obstacle.  QEMU still have too much global
state outside the machine/qdev tree.


>   * Would it be fast enough for libvirt to use?  Do we know if the
>     slowness which makes multiple qemu invocations by libvirt
>     unattractive is from the kernel/libc/ldso overhead, or from qemu's
>     internal start up processing?

My gut feeling is that this could be too slow, if the scope of
"re-exec" is too big.


Now, let me try to go to the opposite extreme: I think you had a
good point in your previous proposal.  Why should we need to
restart/re-execute anything at all just because some bit of
configuration is being changed by libvirt?  Why commands like
set-numa-node should require QEMU to be in a state that is not
covered by -S?  If the guest is not running yet, there should be
no reason to require clients to explicitly pause/continue/restart
anything.

(Translating this to my example above: why exactly have I assumed
above that keeping "query-cpus" working would necessarily make
set-numa-node stop working?)

-- 
Eduardo