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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 11 May 2016 16:00:04 +0200
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> Some systems can already provide more than 255 hardware threads.
> 
> Bumping the QEMU limit to 1024 seems reasonable:
> - it has no visible overhead in top
> - the limit itself has no effect on hot paths
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
> 
> This was tested with a pseries guest only. I'd like to know if it is okay
> for other platforms.
#git grep MAX_CPUMASK_BITS
...
hw/arm/virt.c:    mc->max_cpus = MAX_CPUMASK_BITS;
hw/ppc/spapr.c:    mc->max_cpus = MAX_CPUMASK_BITS;
...

you probably don't want to change all those to 1024,
see/review a similar patch where I bump it upto 288 bits
https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg01101.html


in addition where does 1024 come from?
is pseries capable to provide that many CPUs?

> 
>  include/sysemu/sysemu.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 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 {
>