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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 17 Oct 2017 16:35:15 +0100
"Daniel P. Berrange" <berrange@redhat.com> wrote:

> On Tue, Oct 17, 2017 at 05:21:13PM +0200, Laszlo Ersek wrote:
> > On 10/17/17 16:48, Daniel P. Berrange wrote:  
> > > On Mon, Oct 16, 2017 at 07:01:01PM +0200, Paolo Bonzini wrote:  
> > >> On 16/10/2017 18:59, Eduardo Habkost wrote:  
> > >>>> +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  
> > >>
> > >> Should (or could) "-M none" be changed in a backwards-compatible way to
> > >> allow such preconfiguration?  For example
> > >>
> > >>   qemu -M none -monitor stdio
> > >>   (qemu) machine-set-options pc,accel=kvm
> > >>   (qemu) c  
> > > 
> > > Going down this route has pretty major implications for the way libvirt
> > > manages QEMU, and support / debugging of it. When you look at the QEMU
> > > command line libvirt uses it will be almost devoid of any useful info.
> > > So it will be more involved job to figure out just how QEMU is configured.
> > > This also means it is difficult to replicate the config that libvirt has
> > > used, outside of libvirt for sake of debugging.
> > > 
> > > I also think it will have pretty significant performance implications
> > > for QEMU startup. To configure a guest via the monitor is going to
> > > require a huge number of monitor commands to be executed to replicate
> > > what we traditionally configured via ARGV. While each monitor command
> > > is not massively slow, the round-trip time of each command will quickly
> > > add up to several 100 milliseconds, perhaps even seconds in the the
> > > case of very large configs. 
> > > 
> > > Maybe we ultimately have no choice and this is inevitable, but I am
> > > pretty wary of going in the direction of launching bare QEMU and
> > > configuring everything via a huge number of monitor calls.  
> > 
> > Where's the sweet spot between
> > - configuring everything dynamically, over QMP,
> > - and invoking QEMU separately, for querying capabilities etc?  
> 
> The key with the way we currently invoke & query QEMU over QMP to detect
> capabilities is that this is not tied to a specific VM launch process.
> We can query capabilities and cache them until such time as we detect
> a QEMU binary change. So this never impacts on the startup performance
> of individual VMs. The caching is critical, because querying capabilities
> is actually quite time intensive already, taking many seconds to query
> capabilities on all the different target binaries we have.
is there another alternative for usecase where one option values depends (-numa cpu)
on values of another option values (-M + -smp + -cpu)?
 so far we have 2 options on the table:
   1: do configuration at runtime like in this series
   2: start qemu 2 times
        1st to query cpu layout and
        2nd add -numa options using data from the 1st step

> Regards,
> Daniel