Re: [Qemu-devel] [PATCH] x86: Increase max vcpu number to 352

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Aug 10, 2017 at 06:08:07PM +0800, Lan Tianyu wrote:
> Intel Xeon phi chip will support 352 logical threads. For HPC
> usage case, it will create a huge VM with vcpus number as same as host
> cpus. This patch is to increase max vcpu number to 352.

If we pick arbitray limits based on size of physical CPUs that happen
to be shipping today, we'll continue the cat+mouse game forever trailing
latest CPUs that vendors ship.

IMHO we should pick a higher number influenced by technical constraints
of the q35 impl instead. eg can we go straight to something like 512 or
1024  ?

> Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
> ---
>  hw/i386/pc_q35.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 169a214..5e93749 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -299,7 +299,7 @@ static void pc_q35_machine_options(MachineClass *m)
>      m->default_display = "std";
>      m->no_floppy = 1;
>      m->has_dynamic_sysbus = true;
> -    m->max_cpus = 288;
> +    m->max_cpus = 352;
>  }

You'll need to introduce machine type back compat support so that we
avoid changing the 2.10 q35 machine type - only the 2.11 machine
type should have the new limit.

Regards,
Daniel
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