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

[Thomas Huth <thuth@redhat.com>]

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. Maybe best if you rebase your patch 
on top of:

  https://lore.kernel.org/qemu-devel/20231120094259.1191804-1-cohuck@redhat.com/

   Thomas