Re: [Qemu-devel] [PATCH v1 2/5] vl.c: add -smp, dies=* command line support

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jan 14, 2019 at 06:51:34PM -0200, Eduardo Habkost wrote:
> On Mon, Jan 14, 2019 at 08:24:56PM +0800, Like Xu wrote:
> > This patch updates the check rules on legeacy -smp parse from user command
> > and it's designed to obey the same restrictions as socket/core/thread model.
> > 
> > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> 
> This would require the documentation for -smp to be updated.
> qemu-options.hx still says that "cores=" is the number of cores
> per socket.
> 
> Also, I'm not completely sure we should change the meaning of
> "cores=" and smp_cores to be per-die instead of per-socket.  Most
> machines won't have any code for tracking dies, so we probably
> shouldn't make the extra complexity affect all machines.[1]

Could we not simply have a 'max-dies' property against the machine
base class which defaults to 1. Then no existing machine types
need any changes unless they want to opt-in to supporting
"dies > 1".

> What would be the disadvantages of a simple -machine
> "dies-per-socket" option, specific for PC?

Libvirt currently has

  <cpu>
     <topology sockets='1' cores='2' threads='1'/>
  </cpu>

To me the natural way to expand that is to use

  <cpu>
     <topology sockets='1' dies='2' cores='2' threads='1'/>
  </cpu>

but this rather implies dies-per-socket + cores-per-die
not cores-per-socket.  Libvirt could of course convert
its value from  cores-per-die into cores-per-socket
before giving it to QEMU, albeit with the potential
for confusion from people comparing the libvirt and QEMU
level configs

> Keeping core-id and smp_cores per-socket instead of per-die also
> seems necessary to keep backwards compatibility on the interface
> for identifying CPU hotplug slots.  Igor, what do you think?

Is there really a backwards compatibility problem, given that
no existing mgmt app will have created a VM with "dies != 1".
IOW, if an application adds logic to support configuring a
VM with "dies > 1" it seems fine that they should need to
understand how this impacts the way you identify CPUs for
hotplug.

> [1] I would even argue that the rest of the -smp options belong
>     to the machine object, and topology rules should be
>     machine-specific, but cleaning this up will require
>     additional work.

If we ever expect to support non-homogenous CPUs then our
modelling of topology is fatally flawed, as it doesm't allow
us to specify  creating a VM with  1 socket containing 2
cores and a second socket containing 4 cores. Fixing that
might require modelling each socket, die, and core as a
distinct set of nested QOM objects which gets real fun.


Regards,
Daniel
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