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

[Daniel P. Berrange]

On Tue, Oct 17, 2017 at 05:42:19PM +0200, Laszlo Ersek wrote:
> On 10/17/17 17:35, Daniel P. Berrange 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.
> 
> (Sorry about hijacking the thread, but I can't stop asking :) )
> 
> This looks very smart -- for my own education, how does libvirtd detect
> a QEMU binary change? Based on executable mtime, size, checksum? Are
> perhaps the <emulator> elements of individual domains involved?

We store the capabilities info in an XML file in /var, and this contains
the ctime of libvirtd and or qemu, as well as libvirt version number. If
any of those change, the cache is invalidated.

Regards,
Daniel
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