Re: [Qemu-devel] [PATCH 22/22] add cpu-add qmp command and implement CPU hot-add for target-i386

[Eduardo Habkost]

On Tue, Apr 09, 2013 at 10:19:11PM +0200, Igor Mammedov wrote:
> On Fri, 5 Apr 2013 14:10:54 -0300
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Fri, Apr 05, 2013 at 04:37:16PM +0200, Igor Mammedov wrote:
> > [...]
> > > diff --git a/qapi-schema.json b/qapi-schema.json
> > > index db542f6..a760ed5 100644
> > > --- a/qapi-schema.json
> > > +++ b/qapi-schema.json
> > > @@ -1387,6 +1387,17 @@
> > >  { 'command': 'cpu', 'data': {'index': 'int'} }
> > >  
> > >  ##
> > > +# @cpu-add
> > > +#
> > > +# Adds CPU with specified id
> > > +#
> > > +# @id: cpu id of CPU to be created
> > 
> > Can we have the semantics/constraints of "id" documented here? Is it an
> > arbitrary ID chosen by the caller? Does it have to be the APIC ID? Does
> it's generic function so documenting it as APIC ID is not appropriate.
> 
> I for sure should document it on cpu-hotplug wiki page though, for x86 use
> case for starters. i.e. how to use QMP to get a list of available/free IDs.
> and in which order to use them.
> 
> > it have to be the index of the CPU in the CPU list? How the IDs of
> > existing CPUs set using "-smp" are allocated?
> With current -smp implementation the same way as it was before,
> and for migration to work hot-plugged CPU has to be the next unused APIC
> ID in their sequence, so that target qemu could be started with "-smp n+1".

The problem is that it's hard to find out what's the APIC ID for each
CPU mentioned in the command-line.

For example, if you use "-smp 18,cores=3,threads=3,maxcpus=36" thread ID
will use 2 bits, core ID will use 2 bits, the APIC IDs on startup will
be:

online on startup:
package 0, core 0: 0 1 2
package 0, core 1: 4 5 6
package 0, core 2: 8 9 10
package 1, core 0: 16 17 18
package 1, core 1: 20 21 22
package 1, core 2: 24 25 26

offline on startup:
package 2, core 0: 32 33 34
package 2, core 1: 36 37 38
package 2, core 2: 40 41 42
package 3, core 0: 48 49 50
package 3, core 1: 52 53 54
package 3, core 2: 56 57 58


What should the caller do to find out the correct ID for each of the 36
VCPUs? This should be clearly documented.


> 
> But -smp along with -numa should be reworked to allow specifying guest visible
> CPU IDs for arbitrary CPU hotplug to work.

I'm curious how you plan to make this work while keeping command-line
compatibility. See the question I sent on my other message, about how to
map the IDs used on -numa (that are "CPU indexes") to the IDs required
by cpu-add.


> 
> when we done with QOMifying CPUs it might be possible to use -device for them
> and keeping -smp for compat/shorcut purposes.
> 
> > 
> > I am looking at the code right now to understand how this implementation
> > works, but the documentation could contain or point to documentation on
> > how the "id" parameter is used and interpreted.
> I'll add pointer to wiki and describe there target-i386 use-case.

Thanks! Could you try to document it succintly inside qapi-schema.json
as well? Maybe just a pointer to other documents would be useful.

-- 
Eduardo