Re: [Qemu-arm] [PATCH v2] sysemu: support up to 1024 vCPUs

[Greg Kurz]

On Wed, 18 May 2016 16:31:31 +0100
Peter Maydell <address@hidden> wrote:

> On 18 May 2016 at 14:52, Greg Kurz <address@hidden> wrote:
> > Some PowerPC systems can already provide more than 255 hardware threads.
> > Igor also posted a patch recently, to increase the the maxcpus limit to
> > 288 for x86 targets.
> >
> > This patch bumps the limit to 1024:
> > - 1024 is already the limit for PowerPC KVM in several distros
> > - it has no visible overhead in top
> > - the limit itself has no effect on hot paths
> >
> > We only do that for pseries at the moment and keep a 255 limit for the
> > ARM Virtual Machine.
> >
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> > v2: - updated changelog
> >     - keep 255 limit in hw/arm/virt.c
> > ---
> >  hw/arm/virt.c           |    2 +-
> >  include/sysemu/sysemu.h |    2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > index 56d35c7716e7..438e09c7a57d 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -1358,7 +1358,7 @@ static void virt_machine_class_init(ObjectClass *oc, 
> > void *data)
> >       * it later in machvirt_init, where we have more information about the
> >       * configuration of the particular instance.
> >       */
> > -    mc->max_cpus = MAX_CPUMASK_BITS;
> > +    mc->max_cpus = 255;  
> 
> This change makes the code disagree with the comment immediately
> above it. As the comment says, we set max_cpus to the maximum
> QEMU supports, and then dynamically check it later when we know
> whether we have a GICv2 or GICv3 (which is what puts the limit
> on number of CPUs for this board). So I think the change is
> not needed.
> 

Thank you Peter for the clarification. I'll mention that in the
changelog in v3.

> >      mc->has_dynamic_sysbus = true;
> >      mc->block_default_type = IF_VIRTIO;
> >      mc->no_cdrom = 1;
> > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> > index 38fb3cad35e1..89d742caa477 100644
> > --- a/include/sysemu/sysemu.h
> > +++ b/include/sysemu/sysemu.h
> > @@ -185,7 +185,7 @@ extern int mem_prealloc;
> >   *
> >   * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
> >   */
> > -#define MAX_CPUMASK_BITS 255
> > +#define MAX_CPUMASK_BITS 1024
> >
> >  #define MAX_OPTION_ROMS 16
> >  typedef struct QEMUOptionRom {  
> 
> thanks
> -- PMM
>