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

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

On Thu, Jan 17, 2019 at 09:18:29AM +0800, Like Xu wrote:
> On 2019/1/17 2:26, Daniel P. Berrangé wrote:
> > 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".
> It's nice to have max-dies for machine base class.
> > 
> > > 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
> It is a recommended update on cpu topo configuration of libvirt
> as well as other upper layer apps.
> > 
> > > 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.
> The impacts from MCP model will be documented continuously.
> Any concerns for hot-plugging CPUs in MCP socket is welcomed.
> > 
> > > [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.
> Do we really need to go out of this non-homogeneous step？
> Currently there is no support on physical host AFAIK.
> Is there enough benefit?

I'm not suggesting we need to solve this now - I just meant to indicate
that we shouldn't over-think representing of the 'dies' parameter today,
because any problems with the simple solution you proposed are negligible
compared to the problem of non-homogeneous CPUs. IOW, I think it is fine
to keep your simple proposal now. Worry about the hard problems later
when we'll need better modelling of everything.

Regards,
Daniel
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