Re: [PATCH] pc: q35: Bump max_cpus to 4096

[Ani Sinha]

> On 11-Dec-2023, at 11:27 AM, Thomas Huth <thuth@redhat.com> wrote:
> 
> On 08/12/2023 13.26, Ani Sinha wrote:
>> Since commit f10a570b093e6 ("KVM: x86: Add CONFIG_KVM_MAX_NR_VCPUS to allow 
>> up to 4096 vCPUs")
>> Linux kernel can support upto a maximum number of 4096 vCPUS when MAXSMP is
>> enabled in the kernel. So bump up the max_cpus value for q35 machines 
>> versions
>> 8.3 and newer to 4096. Older q35 machines versions 8.2 and older continue to
>> support 1024 maximum vcpus as before.
>> If KVM is not able to support the specified number of vcpus, QEMU would
>> return the following error messages:
>> $ ./qemu-system-x86_64 -cpu host -accel kvm -machine q35 -smp 4096
>> qemu-system-x86_64: -accel kvm: warning: Number of SMP cpus requested (4096) 
>> exceeds the recommended cpus supported by KVM (12)
>> Number of SMP cpus requested (4096) exceeds the maximum cpus supported by 
>> KVM (1024)
>> Cc: Daniel P. Berrangé <berrange@redhat.com>
>> Cc: Igor Mammedov <imammedo@redhat.com>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Julia Suvorova <jusual@redhat.com>
>> Signed-off-by: Ani Sinha <anisinha@redhat.com>
>> ---
>>  hw/i386/pc_q35.c | 15 ++++++++++++---
>>  1 file changed, 12 insertions(+), 3 deletions(-)
>> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
>> index 4f3e5412f6..2ed57814e1 100644
>> --- a/hw/i386/pc_q35.c
>> +++ b/hw/i386/pc_q35.c
>> @@ -375,7 +375,7 @@ static void pc_q35_machine_options(MachineClass *m)
>>      m->default_nic = "e1000e";
>>      m->default_kernel_irqchip_split = false;
>>      m->no_floppy = 1;
>> -    m->max_cpus = 1024;
>> +    m->max_cpus = 4096;
>>      m->no_parallel = !module_object_class_by_name(TYPE_ISA_PARALLEL);
>>      machine_class_allow_dynamic_sysbus_dev(m, TYPE_AMD_IOMMU_DEVICE);
>>      machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
>> @@ -383,12 +383,22 @@ static void pc_q35_machine_options(MachineClass *m)
>>      machine_class_allow_dynamic_sysbus_dev(m, TYPE_VMBUS_BRIDGE);
>>  }
>>  -static void pc_q35_8_2_machine_options(MachineClass *m)
>> +static void pc_q35_8_3_machine_options(MachineClass *m)
> 
> Hi Ani!
> 
> The next QEMU version after 8.2 is 9.0.

Ah yes I completely forgot about our versioning scheme.


> Maybe best if you rebase your patch on top of:
> 
> https://lore.kernel.org/qemu-devel/20231120094259.1191804-1-cohuck@redhat.com/

Will do. I am trying to get some data with testing on edk2. It seems on legacy 
bios, we have done some testing downstream with 4096 vcpus with qemu 
modifications and patched host kernel [1] and the linux guest boots fine. With 
edk2 it seems we have tested with 1880 vcpus and it works fine with the patched 
edk2 [2]. I am working downstream with HPE folks to get some testing done with 
4096 vcpus.

[1] patched with 
https://github.com/kvm-x86/linux/commit/f10a570b093e60c6bd3f210ae909f014f421352a
 .
[2] 
https://github.com/tianocore/edk2/pull/4181/commits/03ca8224f995eda75e2769ed9a2fc437e321db8e