Re: [Qemu-ppc] [Qemu-devel] [PATCH v2 3/6] exec: set cpu_index only if it's not been explictly set

[Igor Mammedov]

On Tue, 26 Jul 2016 15:28:13 -0300
Eduardo Habkost <address@hidden> wrote:

> On Mon, Jul 25, 2016 at 11:59:21AM +0200, Igor Mammedov wrote:
> > it keeps the legacy behavior for all users that doesn't care
> > about stable cpu_index value, but would allow boards that
> > would support device_add/device_del to set stable cpu_index
> > that won't depend on order in which cpus are created/destroyed.
> > 
> > While at that simplify cpu_get_free_index() as cpu_index
> > generated by USER_ONLY and softmmu variants is the same
> > since none of the users support cpu-remove so far, except
> > of not yet released spapr/x86 device_add/delr, which
> > will be altered by follow up patches to set stable
> > cpu_index manually.  
> 
> So, cpu_get_free_index() behavior is exactly the same because
> cpu-remove is either unsupported, or only supported for the last
> CPU. But I worry that this will easily break if anybody starts
> implementing CPU removal in other machines without setting
> cpu_index explicitly in the board code. Then we can make
> cpu_get_free_index() generate a duplicate cpu_index.
> 
> I wonder if there any way we can add an assert() somewhere to
> ensure no machine will ever allow CPU removal while not
> initializing cpu_index explicitly.
> 
> (This shouldn't hold this patch, it's just a suggestion for a
> possible follow-up patch).
I'll try to post it shortly on top of this patch

> 
> Additional comment:
> 
> > 
> > Signed-off-by: Igor Mammedov <address@hidden>
> > Reviewed-by: David Gibson <address@hidden>  
> [...]
> > -static int cpu_get_free_index(Error **errp)
> > -{
> > -    int cpu = find_first_zero_bit(cpu_index_map, MAX_CPUMASK_BITS);
> > -
> > -    if (cpu >= MAX_CPUMASK_BITS) {
> > -        error_setg(errp, "Trying to use more CPUs than max of %d",
> > -                   MAX_CPUMASK_BITS);
> > -        return -1;
> > -    }  
> 
> We are now relying on the rest of the QEMU code to make sure
> cpu_index will be always < MAX_CPUMASK_BITS. In this case, I
> suggest we add an assert() below:
> 
> [...]
> > -    cpu->cpu_index = cpu_get_free_index(&local_err);
> > -    if (local_err) {
> > -        error_propagate(errp, local_err);
> > -        cpu_list_unlock();
> > -        return;
> > +    if (cpu->cpu_index == UNASSIGNED_CPU_INDEX) {
> > +        cpu->cpu_index = cpu_get_free_index();
> > +        assert(cpu->cpu_index != UNASSIGNED_CPU_INDEX);
> >      }  
> 
> Here:
>   assert(cpu->cpu_index <= MAX_CPUMASK_BITS)
I'd rather get rid of MAX_CPUMASK_BITS but I haven't looked at
how hard it will be yet.

> 
> 
> Both comments can be addressed in a follow-up patch, so:
> 
> Reviewed-by: Eduardo Habkost <address@hidden>
>