Re: [PATCH v4] pc: q35: Bump max_cpus to 4096 vcpus

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 21 Feb 2024 19:32:27 +0530
Ani Sinha <anisinha@redhat.com> 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. At present, QEMU has been tested to correctly boot a
> linux guest with 4096 vcpus with edk2 pending various upstream EDK2 fixes
> which will probably be in the 2024-05 release to be released in the coming
> months. With current seabios firmware, it boots fine with 4096 vcpus already.
> So bump up the value max_cpus to 4096 for q35 machines versions 9 and newer.
> Q35 machines versions 8.2 and older continue to support 1024 maximum vcpus
> as before for compatibility reasons.
> 
> 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>
> Cc: kraxel@redhat.com
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Ani Sinha <anisinha@redhat.com>

From ACPI point of view, it can support upto 4096 logical cpus
(for more we would need to reconsider how to generate CPU descriptions
as well as hotplug).

Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  hw/i386/pc_q35.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Changelog:
> v4: tweaked commit message as per suggestion from danpb explicitly
> stating that 4096 vcpus work with edk2 fixes that are going to be
> available in the coming edk2 release.
> v3: bump up to 4096 vcpus. It has now been tested to work with edk2.
> See RH Jira: https://issues.redhat.com/browse/RHEL-22202
> v2: bump up the vcpu number to 1856. Add failure messages from ekd2 in
> the commit description.
> 
> 
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index d346fa3b1d..ae60e6b919 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -366,7 +366,7 @@ 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);
> @@ -387,6 +387,7 @@ 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);
>  }