Re: [PATCH] pc: q35: Bump max_cpus to 512

[Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>]

On 5/13/22 6:23 PM, Michael S. Tsirkin wrote:
> On Mon, May 09, 2022 at 09:12:49AM +0200, Igor Mammedov wrote:
>> On Wed, 4 May 2022 08:16:39 -0500
>> Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>  wrote:
>>
>>> This is the maximum number of vCPU supported by
>>> the AMD x2APIC virtualization.
>>>
>>> Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@amd.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 302288342a..e82b1c690d 100644
>>> --- a/hw/i386/pc_q35.c
>>> +++ b/hw/i386/pc_q35.c
>>> @@ -357,7 +357,7 @@ static void pc_q35_machine_options(MachineClass *m)
>>>       machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
>>>       machine_class_allow_dynamic_sysbus_dev(m, TYPE_RAMFB_DEVICE);
>>>       machine_class_allow_dynamic_sysbus_dev(m, TYPE_VMBUS_BRIDGE);
>>> -    m->max_cpus = 288;
>>> +    m->max_cpus = 512;
>> Maybe we should bump it to KVM VCPU maximum,
>> and make sure we error out if asked for combination of
>> hardware/irqchip is not usable.
>
> So what happens if one does 710 and then tries to use AMD x2APIC?
> We'd like that to error out, right?
> 

Since the AMD SVM x2AVIC can support only upto 512 vCPUs,
KVM would return error when QEMU tries to create more than 512 vcpus
in the following call-path:

   kvm_vm_ioctl_create_vcpu
     kvm_arch_vcpu_create
       svm_vcpu_create
         avic_init_vcpu

Also, I tried to find a way to get the KVM VCPU max value via KVM_CAP_MAX_VCPUS ioctl,
but the accel/kvm/kvm-all.c:kvm_init() is called later than hw/core/machine-smp.c:
machine_parse_smp_config(), where it checks the specified QEMU -smp option.

So, if there is no objection, I will send out a patch to change m->max_cpus = 1024
to match current KVM limit.

Best Regards,
Suravee