Re: [Qemu-devel] [PATCH] docs: add cpu-hotplug.txt

[David Gibson]

On Mon, Aug 08, 2016 at 02:18:05PM +0200, Igor Mammedov wrote:
> On Mon, 8 Aug 2016 10:28:02 +0800
> Dou Liyang <address@hidden> wrote:
> 
> > This document describes how to use cpu hotplug in QEMU.
> > 
> > Signed-off-by: Dou Liyang <address@hidden>
> > ---
> >  docs/cpu-hotplug.txt | 110 
> > +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 110 insertions(+)
> >  create mode 100644 docs/cpu-hotplug.txt
> > 
> > diff --git a/docs/cpu-hotplug.txt b/docs/cpu-hotplug.txt
> > new file mode 100644
> > index 0000000..d62638e
> > --- /dev/null
> > +++ b/docs/cpu-hotplug.txt
> > @@ -0,0 +1,110 @@
> > +QEMU CPU hotplug
> > +===================
> > +
> > +This document explains how to use the cpu hotplug feature in QEMU,
> > +which is present since v2.6.0.
> 
>  -device/device_add based CPUs are merged since 2.7
> 
> > +
> > +Guest support is required for cpu hotplug to work.
> > +
> > +CPU hotplug
> > +-------------------
> > +
> > +In order to be able to hotplug cpu, QEMU has to be told what is the
> > +maximum amount of cpus the guest can grow. This is done at startup
> > +time by means of the -smp command-line option, which has the following
> > +format:
> > +
> > + -smp [cpus=]n[,maxcpus=cpus][,cores=cores][,threads=threads]
> > +   [,sockets=sockets]
> > +
> > +Where,
> > +
> > + - "cpus" set the number of CPUs to 'n' [default=1]
> > + - "maxcpus" maximum number of total cpus, including offlineCPUs for
> > +   hotplug, etc
> s/total//
> s/offlineCPUs/offline VCPUs/ or threads
> 
> > + - "cores" number of CPU cores on one socket
> > + - "threads= number of threads on one CPU core
> > + - "sockets= number of discrete sockets in the system
> Above needs additional meaning clarification for SPAPR,
> CCing David

Sockets have no real meaning on sPAPR.  You can decide how many cores
go into each "socket", but it has no real effect on the guest.

> 
> > +
> > +
> > +For example, the following command-line:
> > +
> > + qemu [...] -smp 3,maxcpus=10,sockets=2,cores=2,threads=2
> > +
> > +Creates a guest with 3 cpus and it support up to 10 cpus. The cpu
> > +topology is sockets (2) * cores (2) * threads (2) and can't greater
> > +than maxcpus. When the guest is just booted, the guest will see 3
> > +cpus. so there are seven cpus can be hotplugged by using any
> > +combination of the available sockets,cores and threads topology or
> > +using apic-id.
> s/cpus/vcpus/
> 
> drop any mentioning of apic-id, it's not supposed to be used
> by user and socket/core/thread-ids should be used i.e.
> properties advertised by QEMU via QMP command query-hotpluggable-cpus
> or corresponding HMP command 'info hotpluggable-cpus'
> 
> PS:
> Eduardo should we drop apic-id property before 2.7 is released?
> So people won't even try to use it, it wan never a public
> property before as cpus weren't available with -device/device_add.
> 
> 
> > +cpu hot-plug
> > +-----------------------
> > +
> > +A monitor commands are used to hotplug cpu:
> > +
> > + - "device_add": creates a cpu device and inserts it into the
> > + specific topology as a device
> > +
> > +For example, the following commands add a cpu which id is cpu1 to
> > +the guest discussed earlier:
> > +
> > +  (qemu) device_add qemu64-x86_64-cpu,id=cpu1,apic-id=3
> likewise remove any mentioning of apic-id from document
> 
> I'd add as the first section here commands that query possible
> available cpus including topology properties. 
> 
> > +
> > + - "qemu64-x86_64-cpu" is the cpu modle.
> > + - "id" is the unique identifier in the device sets.
> > + - "apic-id" is the hotpluged cpu's physical identification.
> > +
> > +Another command uses the cpu topology to add the additional cpu in
> > +the designated position.
> > +
> > +For example, the following commands add a cpu in the last position
> > +of the guest cpu topology discussed earlier.
> > +
> > +  (qemu) device_add qemu64-x86_64-cpu,id=cpu1,socket-id=2,core-id=1,
> > +   thread-id=1
> > +
> > +It's also possible to start a guest with cpu cold-plugged into the
> > +hotpluggable cpu topology.
> s/hotpluggable cpu topology/specific place (socket,core,thread)/
> 
> > +
> > +In the following command-line example, a guest which has 3 cpus is
> > +created where one of the cpus comes from the "apic-id", and another
> > +one comes from "socket-id...". After that, the guest has additional
> > +seven cpus to be hot-plug when needed:
> > +
> > + qemu  [...] -smp 1,maxcpus=10,sockets=2,cores=2,threads=2
> > +   -device qemu64-x86_64-cpu,id=cpu1,apic-id=1
> > +   -device qemu64-x86_64-cpu,socket-id=2,core-id=1,thread-id=0
> > +
> > +cpu hot-unplug
> > +------------------------
> > +
> > +In order to be able to hot unplug cpu device, QEMU has two ways
> > +to remove cpu device.
> > +  1. Using the ids which were assigned when you hot plugged cpus.
> > +  2. Using qom_path where the cpu is located in the guest.
> I wouldn't advertise #2 as it's possible to unplug CPUs created with
> -smp X with its help and then have broken migration
> 
> device_del qom_path should even be able to remove a device without ID,
> it should allow removal of devices below /machine/peripheral only
> instead of current anywhere.
> 
> Daniel,
> is it a bug in current device_del impl.?
>  i.e. device_del /machine/unattached/device[2]
>  is currently allowed while I think it shouldn't.
> 
> > +
> > +A monitor commands are used to hot unplug cpus:
> > +
> > + - "device_del": deletes a cpu device
> > +
> > +For example, assuming that the cpu device with id "cpu1" exists,
> > +the following commands tries to remove it.
> > +
> > +  (qemu) device_del cpu1
> > +
> > +If you don't set the ids when you hot plugged cpus.
> > +
> 
> 
> > +First, you may need to obtain the cpu's qom_path. The following
> > +commands list all hotplugable-cpus, you can get the qom_path
> > +through the cpu topology.
> > +
> > +  (qemu) info hotpluggable-cpus
> > +
> > +Then you can remove it by the qom_path.
> > +
> > +For example, assuming that the cpu device with qom_path "/machine
> > +/unattached/device[0]" exists, the following commands tries to
> > +remove it.
> > +
> > +  (qemu) device_del /machine/unattached/device[0]
> I'd remove qom-path related parts and leave only ID based one.
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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