Re: [Qemu-ppc] [PATCH v7 01/13] machine: Don't allow CPU toplogies with partially filled cores

[Eduardo Habkost]

On Fri, Jan 29, 2016 at 02:52:30PM +1100, David Gibson wrote:
> On Thu, Jan 28, 2016 at 11:19:43AM +0530, Bharata B Rao wrote:
> > Prevent guests from booting with CPU topologies that have partially
> > filled CPU cores or can result in partially filled CPU cores after
> > CPU hotplug like
> > 
> > -smp 15,sockets=1,cores=4,threads=4,maxcpus=16 or
> > -smp 15,sockets=1,cores=4,threads=4,maxcpus=17.
> > 
> > This is enforced by introducing MachineClass::validate_smp_config()
> > that gets called from generic SMP parsing code. Machine type versions
> > that want to enforce this can define this to the generic version
> > provided.
> > 
> > Only sPAPR and PC machine types starting from version 2.6 enforce this in
> > this patch.
> > 
> > Signed-off-by: Bharata B Rao <address@hidden>
> 
> I've been kind of lost in the back and forth about
> threads/cores/sockets.
> 
> What, in the end, is the rationale for allowing partially filled
> sockets, but not partially filled cores?

I don't think there's a good reason for that (at least for PC).

It's easier to relax the requirements later if necessary, than
dealing with compatibility issues again when making the code more
strict. So I suggest we make validate_smp_config_generic() also
check if smp_cpus % (smp_threads * smp_cores) == 0.

-- 
Eduardo