[Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Dou Liyang]

This document describes how to use cpu hotplug in QEMU.

Change log v1:
  From Igor's advice:
    1. Remove any mentioning of apic-id from the document.
    2. Remove the "device_del qom_path" from the CPU hot-unplug.
    3. Fix some comment.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
---
 docs/cpu-hotplug.txt | 124 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 124 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..2c51e27
--- /dev/null
+++ b/docs/cpu-hotplug.txt
@@ -0,0 +1,124 @@
+QEMU CPU hotplug
+===================
+
+This document explains how to use the CPU hotplug feature in QEMU,
+which is present since v2.7.
+
+Guest support is required for CPU hotplug to work.
+
+CPU hot-plug
+-------------------
+
+In order to be able to hotplug CPUs, 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 CPUs, including offline VCPUs for
+	hotplug, etc
+ - "cores" number of CPU cores on one socket
+ - "threads= number of threads on one CPU core
+ - "sockets= number of discrete sockets in the system(on sPAPR,
+	sockets have no real meaning, 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 VCPUs and it support up to 10 VCPUs. 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
+VCPUs.
+
+There are seven VCPUs can be hotplugged by socket/core/thread-ids
+which should be obtained from the properties advertised by QEMU via
+the QMP command query-hotpluggable-cpus or the corresponding HMP
+command "info hotpluggable-cpus".
+
+Query possible available CPU objects
+--------------------------------------
+
+Before add the VCPUs, we should know the topology properties of the
+possible available CPUs objects, so that we can find out the available
+place (socket,core,thread) for a new VCPU.
+
+A monitor commands are used to list the possible CPU objects:
+
+  (qemu) info hotpluggable-cpus
+
+Select the hotpluggable CPUs including "CPUInstance Properties" for
+hotpluging. Such as this:
+
+  ...
+  type: "qemu64-x86_64-cpu"
+  vcpus_count: "1"
+  CPUInstance Properties:
+    socket-id: "0"
+    core-id: "1"
+    thread-id: "0"
+  ...
+
+Hotplug CPUs
+----------------------
+
+A monitor commands are used to hotplug CPUs:
+
+ - "device_add": creates a VCPU device and inserts it into the
+	specific place as a device
+
+For example, the following commands add a VCPU which id is cpu1 in
+the last position of the guest's CPU sockets which was discussed
+earlier by using the socket/core/thread-ids:
+
+  (qemu) device_add qemu64-x86_64-cpu,id=cpu9,socket-id=2,core-id=0,
+	thread-id=1
+
+Where,
+
+ - "qemu64-x86_64-cpu" is the CPU modle.
+ - "id" is the unique identifier in the device sets. there must have
+	it.
+ - "socket-id/core-id/thread-id" is represented the designated position
+ which are gained form the above possible list of CPUs.
+
+It's also possible to start a guest with cpu cold-plugged into the
+specific place (socket,core,thread).
+
+In the following command-lines example, a guest which has 3 VCPUs is
+created:
+
+ qemu  [...] -smp 1,maxcpus=10,sockets=2,cores=2,threads=2 \
+	-device qemu64-x86_64-cpu,id=cpu1,socket-id=1, \
+	core-id=1,thread-id=0 \
+	-device qemu64-x86_64-cpu,id=cpu2,socket-id=1, \
+	core-id=1,thread-id=1 \
+
+Two VCPUs are cold-plugged by "-device" parameter. They are in the
+same socket and core with different thread-id. After that, the guest
+has additional seven VCPUs to be hot-plugged when needed.
+
+CPU hot-unplug
+------------------------
+
+In order to be able to hot unplug cpu device, QEMU has the way
+to remove cpu device.
+
+  Using the ids which were assigned when you hot plugged cpus.
+
+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
+
-- 
2.5.5

Re: [Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Fam Zheng]

Hello Liyang, Thanks for the contribution!

On Tue, 08/09 14:48, Dou Liyang wrote:
> This document describes how to use cpu hotplug in QEMU.
> 
> Change log v1:
>   From Igor's advice:
>     1. Remove any mentioning of apic-id from the document.
>     2. Remove the "device_del qom_path" from the CPU hot-unplug.
>     3. Fix some comment.

In the future, please put the revision log after the "---" line below your
signed-off-by line, so that it doesn't get committed to the git history (this
information is not useful there) once get merged.

I'm not a native English speaker either, but I'll try to give some suggestions
on the documentation below. Please don't mind, and take with a grain of salt.  :)

> 
> Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
> ---
>  docs/cpu-hotplug.txt | 124 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 124 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..2c51e27
> --- /dev/null
> +++ b/docs/cpu-hotplug.txt
> @@ -0,0 +1,124 @@
> +QEMU CPU hotplug
> +===================

I think a prettier way to write markdown style docs is to let the underlines be
as long as the text, like:

QEMU CPU hotplug
================

The same applies to following section titles as well.

> +
> +This document explains how to use the CPU hotplug feature in QEMU,
> +which is present since v2.7.

This isn't quite accurate, cpu-add has been available since 1.5 according to
http://wiki.qemu.org/Features/CPUHotplug, I guess you are talking about
something different. So you probably want to revise the sentence to clarify.

> +
> +Guest support is required for CPU hotplug to work.
> +
> +CPU hot-plug
> +-------------------
> +
> +In order to be able to hotplug CPUs, QEMU has to be told what is the

s/what is//, or put "is" after "grow".

> +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]

Not very consistent in the syntax used in this list. There is a verb in "cpus"
but none in the others. I suggest adding a verb to each item.

> + - "maxcpus" maximum number of CPUs, including offline VCPUs for
> +	hotplug, etc
> + - "cores" number of CPU cores on one socket
> + - "threads= number of threads on one CPU core
> + - "sockets= number of discrete sockets in the system(on sPAPR,

s/system(on/system (on)/

> +	sockets have no real meaning, 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 VCPUs and it support up to 10 VCPUs. The CPU

s/support/supports/

> +topology is sockets (2) * cores (2) * threads (2) and can't greater

s/greater/be greater/

> +than maxcpus. When the guest is just booted, the guest will see 3

s/just//

> +VCPUs.
> +
> +There are seven VCPUs can be hotplugged by socket/core/thread-ids

s/There are seven/Seven more/

> +which should be obtained from the properties advertised by QEMU via

s/should be obtained/can be observed/

> +the QMP command query-hotpluggable-cpus or the corresponding HMP
> +command "info hotpluggable-cpus".

This paragraph seems to be repeating what is following. Maybe be more brief and
say "more on this below"?

> +
> +Query possible available CPU objects
> +--------------------------------------
> +
> +Before add the VCPUs, we should know the topology properties of the

s/add/adding/

> +possible available CPUs objects, so that we can find out the available
> +place (socket,core,thread) for a new VCPU.
> +
> +A monitor commands are used to list the possible CPU objects:

s/commands are used/command can be used/

> +
> +  (qemu) info hotpluggable-cpus
> +
> +Select the hotpluggable CPUs including "CPUInstance Properties" for
> +hotpluging. Such as this:
> +
> +  ...
> +  type: "qemu64-x86_64-cpu"
> +  vcpus_count: "1"
> +  CPUInstance Properties:
> +    socket-id: "0"
> +    core-id: "1"
> +    thread-id: "0"
> +  ...
> +
> +Hotplug CPUs
> +----------------------
> +
> +A monitor commands are used to hotplug CPUs:

s/commands are used/command can be used/

> +
> + - "device_add": creates a VCPU device and inserts it into the
> +	specific place as a device
> +
> +For example, the following commands add a VCPU which id is cpu1 in

s/commands add/command adds/
s/which id is/which has id/

> +the last position of the guest's CPU sockets which was discussed
> +earlier by using the socket/core/thread-ids:
> +
> +  (qemu) device_add qemu64-x86_64-cpu,id=cpu9,socket-id=2,core-id=0,
> +	thread-id=1
> +
> +Where,
> +
> + - "qemu64-x86_64-cpu" is the CPU modle.

s/modle/model/

> + - "id" is the unique identifier in the device sets. there must have
> +	it.

s/there must have it/It is required./

> + - "socket-id/core-id/thread-id" is represented the designated position
> + which are gained form the above possible list of CPUs.

s/is represented/represent/
s/are gained/is obtained/


> +
> +It's also possible to start a guest with cpu cold-plugged into the
> +specific place (socket,core,thread).
> +
> +In the following command-lines example, a guest which has 3 VCPUs is
> +created:
> +
> + qemu  [...] -smp 1,maxcpus=10,sockets=2,cores=2,threads=2 \
> +	-device qemu64-x86_64-cpu,id=cpu1,socket-id=1, \
> +	core-id=1,thread-id=0 \
> +	-device qemu64-x86_64-cpu,id=cpu2,socket-id=1, \
> +	core-id=1,thread-id=1 \
> +
> +Two VCPUs are cold-plugged by "-device" parameter. They are in the
> +same socket and core with different thread-id. After that, the guest

s/thread-id/thread-ids/

> +has additional seven VCPUs to be hot-plugged when needed.
> +
> +CPU hot-unplug
> +------------------------
> +
> +In order to be able to hot unplug cpu device, QEMU has the way
> +to remove cpu device.
> +
> +  Using the ids which were assigned when you hot plugged cpus.

Why the indentation for this line?

> +
> +A monitor commands are used to hot unplug cpus:

s/commands are used/command can be used/

> +
> + - "device_del": deletes a cpu device
> +
> +For example, assuming that the cpu device with id "cpu1" exists,
> +the following commands tries to remove it.

s/commands/command/

> +
> +  (qemu) device_del cpu1
> +

Can you also document what should be done in guest to make this operation
successful?

Fam

Re: [Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Dou Liyang]

Hi Fam,

在 2016年08月09日 15:14, Fam Zheng 写道:
> Hello Liyang, Thanks for the contribution!
>
> On Tue, 08/09 14:48, Dou Liyang wrote:
>> This document describes how to use cpu hotplug in QEMU.
>>
>> Change log v1:
>>   From Igor's advice:
>>     1. Remove any mentioning of apic-id from the document.
>>     2. Remove the "device_del qom_path" from the CPU hot-unplug.
>>     3. Fix some comment.
>
> In the future, please put the revision log after the "---" line below your
> signed-off-by line, so that it doesn't get committed to the git history (this
> information is not useful there) once get merged.
>
> I'm not a native English speaker either, but I'll try to give some suggestions
> on the documentation below. Please don't mind, and take with a grain of salt.  :)
>

Thank you very much for your advice. They are very helpful for me. :)

>>
>> Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
>> ---
>>  docs/cpu-hotplug.txt | 124 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 124 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..2c51e27
>> --- /dev/null
>> +++ b/docs/cpu-hotplug.txt
>> @@ -0,0 +1,124 @@
>> +QEMU CPU hotplug
>> +===================
>

>> +For example, assuming that the cpu device with id "cpu1" exists,
>> +the following commands tries to remove it.
>
> s/commands/command/
>
>> +
>> +  (qemu) device_del cpu1
>> +
>
> Can you also document what should be done in guest to make this operation
> successful?
>

I am not sure about these words "what should be done in guest".

Do you mean that setting the CPUx's online to 1 to make the CPUx
online? or anything else?

> Fam
>
>

Thanks,

Dou Liyang

Re: [Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Fam Zheng]

On Tue, 08/09 16:47, Dou Liyang wrote:
> >>+
> >>+  (qemu) device_del cpu1
> >>+
> >
> >Can you also document what should be done in guest to make this operation
> >successful?
> >
> 
> I am not sure about these words "what should be done in guest".
> 
> Do you mean that setting the CPUx's online to 1 to make the CPUx
> online? or anything else?

Yes, like that, but s/online/offline/ ? :)

Fam

Re: [Qemu-devel] [PATCH v1] docs: add cpu-hotplug.txt

[Dou Liyang]

Hi Fam

在 2016年08月09日 17:07, Fam Zheng 写道:
> On Tue, 08/09 16:47, Dou Liyang wrote:
>>>> +
>>>> +  (qemu) device_del cpu1
>>>> +
>>>
>>> Can you also document what should be done in guest to make this operation
>>> successful?
>>>
>>
>> I am not sure about these words "what should be done in guest".
>>
>> Do you mean that setting the CPUx's online to 1 to make the CPUx
>> online? or anything else?
>
> Yes, like that, but s/online/offline/ ? :)
>

OK,

But, I have a question.
In QEMU, when I remove a VCPU which is still online, I also can remove
it successful.


Thanks,

Dou Liyang