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

[Lan Tianyu]

On 2017年08月10日 18:26, Daniel P. Berrange wrote:
> 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  ?

Sure. 512 should be enough and some arrays is defined according to max
vcpu number.

> 
>> Signed-off-by: Lan Tianyu <address@hidden>
>> ---
>>  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.

How about the following change?

-static void pc_q35_2_10_machine_options(MachineClass *m)
+static void pc_q35_2_11_machine_options(MachineClass *m)
 {
      pc_q35_machine_options(m);
      m->alias = "q35";
+}
+
+static void pc_q35_2_10_machine_options(MachineClass *m)
+{
+    pc_q35_2_11_options(m);
+    m->alias = "q35";
+    m->max_cpus = 288;
     m->numa_auto_assign_ram = numa_legacy_auto_assign_ram;
 }



> 
> Regards,
> Daniel
> 


-- 
Best regards
Tianyu Lan