Re: [PATCH v3] pc: q35: Bump max_cpus to 4096 vcpus

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

On Tue, Feb 20, 2024 at 09:12:04PM +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. At present, QEMU has been tested to correctly boot a
> linux guest with 4096 vcpus both with edk2 and seabios firmwares.
> 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
> Signed-off-by: Ani Sinha <anisinha@redhat.com>
> ---
>  hw/i386/pc_q35.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Changelog:
> 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

That bug indicates a dependancy on a EDK2 patch

  https://github.com/kraxel/edk2/commit/7a03c17f0f4f4a9003d77db2660c8e087604b2f0

we'll need to rebase the EDK2 ROMs in QEMU to get that included.

Meanwhile, plesae at least call out this EDK2 commit as a
pre-requisite in the commit message, so people know the
EDK2 ROMs in QEMU won't work (yet).

> 
> 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);
>  }

Assuming tweaked commit message

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|