Re: [Qemu-trivial] [Qemu-devel] [PATCH] vl: remove (max_cpus > 255) check from smp_parse

[Eduardo Habkost]

On Wed, Dec 04, 2013 at 04:50:59PM +1100, Alexey Kardashevskiy wrote:
> On 12/04/2013 01:47 AM, Eduardo Habkost wrote:
> > On Tue, Dec 03, 2013 at 02:30:48PM +0100, Andreas Färber wrote:
> >> Am 03.12.2013 00:03, schrieb Alexey Kardashevskiy:
> >>> On 12/03/2013 09:09 AM, Andreas Färber wrote:
> >>>> Am 02.12.2013 18:06, schrieb Michael Tokarev:
> >>>>> 25.11.2013 07:39, Alexey Kardashevskiy wrote:
> >>>>>> Since modern POWER7/POWER8 chips can have more that 256 CPU threads
> >>>>>> (>2000 actually), remove this check from smp_parse.
> >>>>>>
> >>>>>> The CPUs number is still checked against machine->max_cpus and this 
> >>>>>> check
> >>>>>> should be enough not to break other archs.
> >>>>
> >>>> "should be" is not exactly the highest level of confidence for a
> >>>> "trivial" patch... :/
> >> [...]
> >>>> Alexey, did you actually check that, e.g., x86 machines don't break with
> >>>> 256 or 257 CPUs now?
> >>>
> >>> PC_DEFAULT_MACHINE_OPTIONS sets it to 255. And I cannot find any machine
> >>> which would not define max_cpus, have I missed any?
> >>
> >> If you've actually *checked* the other machines' code then fine with me,
> >> just say so in the commit message. :)
> > 
> > I just grepped for "max_cpus" and checked every match. The largest
> > values I found were:
> > 
> > hw/ppc/spapr.c: 256
> > s390: 255
> > pc: 255
> > 
> > All the rest had values <= 32.
> > 
> > Machines with missing max_cpus value shouldn't be a problem, as
> > max_cpus==0 is interpreted as 1 by the vl.c code.
> > 
> > But we still need to add a check for max_cpus > machine->max_cpus to
> > vl.c, before we eliminate the smp_parse() check.
> 
> 
> Since smp_parse() checks if (max_cpus >= smp_cpus), this should just work:
> 
> diff --git a/vl.c b/vl.c
> index e6ed260..544165a 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -3882,9 +3882,9 @@ int main(int argc, char **argv, char **envp)
>      smp_parse(qemu_opts_find(qemu_find_opts("smp-opts"), NULL));
> 
>      machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */
> -    if (smp_cpus > machine->max_cpus) {
> +    if (max_cpus > machine->max_cpus) {
>          fprintf(stderr, "Number of SMP cpus requested (%d), exceeds max cpus 
> "
> -                "supported by machine `%s' (%d)\n", smp_cpus,  machine->name,
> +                "supported by machine `%s' (%d)\n", max_cpus,  machine->name,
>                  machine->max_cpus);
>          exit(1);
>      }
> 
> 
> > There's also this, at main():
> > 
> >         if (i == nb_numa_nodes) {
> >             for (i = 0; i < max_cpus; i++) {
> >                 set_bit(i, node_cpumask[i % nb_numa_nodes]);
> >             }
> >         }
> > 
> > node_cpumask[] is initialized using bitmap_new(MAX_CPUMASK_BITS), and
> > MAX_CPUMASK_BITS is 255. To fix this, we can initialize node_cpumask[] using
> > max_cpus instead, if we initialize it after smp_parse().
> 
> 
> Nope. At the moment when we parse -numa in vl.c, we may not know yet what
> machine is going to be used and machines can have different max_cpus.

This will be changed by:

  Subject: [PATCH V17 04/11] NUMA: convert -numa option to use OptsVisitor
  Message-Id: <address@hidden>
  http://article.gmane.org/gmane.comp.emulators.qemu/244826

> 
> For now, I would simply change MAX_CPUMASK_BITS to something crazy, like
> 16384 (2KB per numa node), I hope QEMU can survive such a memory waste :)
> 
> Ok?

I'm OK with that as long the code has proper checks in case max_cpus
gets set to a crazily large value (larger than MAX_CPUMASK_BITS) in the
far future, or if we prevent max_cpus from being larger than
MAX_CPUMASK_BITS.

-- 
Eduardo