Re: [PATCH] pc: q35: Bump max_cpus to 1728 vcpus

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

On Tue, Jan 30, 2024 at 10:14:28PM +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. QEMU has been tested to correctly boot a linux guest
> with 1728 vcpus both with edk2 and seabios firmwares. So bump up the max_cpus
> value for q35 machines versions 9 and newer to 1728. Q35 machines versions
> 8.2 and older continue to support 1024 maximum vcpus as before for
> compatibility.

Where does the 1728 number come from ?

Did something break at 1729, or did the test machine simply not
have sufficient resources to do practical larger tests ?

> 
> 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 1728
> qemu-system-x86_64: -accel kvm: warning: Number of SMP cpus requested (1728) exceeds the recommended cpus supported by KVM (12)
> qemu-system-x86_64: -accel kvm: warning: Number of hotpluggable cpus requested (1728) exceeds the recommended cpus supported by KVM (12)
> Number of SMP cpus requested (1728) 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 | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index f43d5142b8..bfa627a70b 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 = 1728;
>      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);
> @@ -396,6 +396,7 @@ static void pc_q35_8_2_machine_options(MachineClass *m)
>  {
>      pc_q35_9_0_machine_options(m);
>      m->alias = NULL;
> +    m->max_cpus = 1024;
>      compat_props_add(m->compat_props, hw_compat_8_2, hw_compat_8_2_len);
>      compat_props_add(m->compat_props, pc_compat_8_2, pc_compat_8_2_len);
>  }
> -- 
> 2.42.0
> 

With regards,
Daniel
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