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

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

On Fri, Dec 08, 2023 at 05:56:11PM +0530, 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(-)

What testing has been done to confirm if QEMU is actually capable of
booting a guest with this CPU count, either UEFI or SeaBIOS or both ?

Historically every time we wanted to raise max cpus we've seen limits
or scalability problems that needed fixing first. The previous bump to
1024 had been implicitly proven via downstream testing we had done in
RHEL, and had required the switch to SMBIOS v3 entrypoint.

> 
> 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)
>  {
>      pc_q35_machine_options(m);
>      m->alias = "q35";
>  }
>  
> +DEFINE_Q35_MACHINE(v8_3, "pc-q35-8.3", NULL,
> +                   pc_q35_8_3_machine_options);
> +
> +static void pc_q35_8_2_machine_options(MachineClass *m)
> +{
> +    pc_q35_8_3_machine_options(m);
> +    m->alias = NULL;
> +    m->max_cpus = 1024;
> +}
> +
>  DEFINE_Q35_MACHINE(v8_2, "pc-q35-8.2", NULL,
>                     pc_q35_8_2_machine_options);
>  
> @@ -396,7 +406,6 @@ static void pc_q35_8_1_machine_options(MachineClass *m)
>  {
>      PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_q35_8_2_machine_options(m);
> -    m->alias = NULL;
>      pcmc->broken_32bit_mem_addr_check = true;
>      compat_props_add(m->compat_props, hw_compat_8_1, hw_compat_8_1_len);
>      compat_props_add(m->compat_props, pc_compat_8_1, pc_compat_8_1_len);
> -- 
> 2.42.0
> 

With regards,
Daniel
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