Re: [Qemu-devel] [PATCH v2 03/24] hw/arm/virt: use machine->possible_cpus for storing possible topology info

[Andrew Jones <drjones@redhat.com>]

On Thu, May 04, 2017 at 02:55:09PM +0200, Igor Mammedov wrote:
> On Thu, 4 May 2017 11:38:22 +0200
> Andrew Jones <drjones@redhat.com> wrote:
> 
> > On Wed, May 03, 2017 at 02:56:57PM +0200, Igor Mammedov wrote:
> > > for now precalculate and store mp_afinity in possible_cpus
> > > as ARM cpus don't have socket/core/thread-id properties yet.
> > > In follow patches possible_cpus will be used for storing
> > > and setting NUMA node mapping and replace legacy bitmap
> > > based numa_info[node_id].node_cpu/numa_get_node_for_cpu()
> > > 
> > > For the lack of better idea, this patch cannibalizes
> > > possible_cpus.cpus[x].props.thread_id so that
> > > *_cpu_index_to_props() callback could return addressable
> > > by props CPU which will be used by machine_set_cpu_numa_node()
> > > in follow up patches to assign a CPU to node. But
> > > cannibalizing is fine for now as that thread_id isn't exposed
> > > to users (no hotpluggable_cpus callback support for ARM yet)
> > > and it will be used only internally until 'device_add cpu'
> > > is supported where we can decide on which properties to use.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > > v2:
> > >   (Drew)
> > >     - discarding result of possible_cpu_arch_ids() makes
> > >       call not obvious and is confusing. Instead assign
> > >       possible_cpu_arch_ids() result to local var and use
> > >       it instead of direct access to machine->possible_cpus
> > >       field, as it's done in pc.c
> > > ---
> > >  hw/arm/virt.c | 40 +++++++++++++++++++++++++++++++++++++---
> > >  1 file changed, 37 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > > index 61ae437..e2c5626 100644
> > > --- a/hw/arm/virt.c
> > > +++ b/hw/arm/virt.c
> > > @@ -1221,6 +1221,8 @@ static void machvirt_init(MachineState *machine)
> > >  {
> > >      VirtMachineState *vms = VIRT_MACHINE(machine);
> > >      VirtMachineClass *vmc = VIRT_MACHINE_GET_CLASS(machine);
> > > +    MachineClass *mc = MACHINE_GET_CLASS(machine);
> > > +    const CPUArchIdList *possible_cpus;
> > >      qemu_irq pic[NUM_IRQS];
> > >      MemoryRegion *sysmem = get_system_memory();
> > >      MemoryRegion *secure_sysmem = NULL;
> > > @@ -1344,10 +1346,16 @@ static void machvirt_init(MachineState *machine)
> > >          exit(1);
> > >      }
> > >  
> > > -    for (n = 0; n < smp_cpus; n++) {
> > > -        Object *cpuobj = object_new(typename);
> > > +    possible_cpus = mc->possible_cpu_arch_ids(machine);
> > > +    for (n = 0; n < possible_cpus->len; n++) {
> > > +        Object *cpuobj;
> > >  
> > > -        object_property_set_int(cpuobj, virt_cpu_mp_affinity(vms, n),
> > > +        if (n >= smp_cpus) {
> > > +            break;
> > > +        }  
> > 
> > Why the break instead of just looping 'n < smp_cpus' like x86 does? Is
> > there some future work where looping up to possible_cpus->len (aka
> > max_cpus) is what we'll eventually want? If so, then we need a TODO
> > comment here. If not, then we should clean this up by removing the break.
> There is no plans to loop here upto possible_cpus->len.
> 
> It seemed to me more consistent/safer to use index limited
> by possible_cpus->len to index possible_cpus->cpus[n] array
> than index limited by smp_cpus though the former currently is
> always less than smp_cpus.
         ^ greater than or equal to
> 
> If you prefer 'n < smp_cpus' loop, then I can switch to it.

I just don't like the 'if (n >= smp_cpus) { break; }' - the whole thing
would look much nicer without it. And, if there's a valid concern that
possible_cpus->len can be < smp_cpus, then we should check it in x86
too. Anyway we can check both conditions in the 'for', which would
look a bit more pleasing to me...

 for (n = 0; n < possible_cpus->len && n < smp_cpus; n++) {
     Object *cpuobj = object_new(typename);
     object_property_set_int(cpuobj, possible_cpus->cpus[n].arch_id,
                             "mp-affinity", NULL);
     ...

All that said, it's just a nit in the end, so

Reviewed-by: Andrew Jones <drjones@redhat.com>