Re: [PATCH v5] pc: q35: Bump max_cpus to 4096 vcpus

[Ani Sinha]

> On 28-Feb-2024, at 20:03, 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 using the current edk2 upstream master branch that
> has the fixes corresponding to the following two PRs:
> 
> https://github.com/tianocore/edk2/pull/5410
> https://github.com/tianocore/edk2/pull/5418
> 
> The changes merged into edk2 with the above PRs will be in the upcoming 
> 2024-05
> release. 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>
> Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> Signed-off-by: Ani Sinha <anisinha@redhat.com>

Ping .. who is picking this up? The soft code freeze starts today?

> ---
> hw/i386/pc_q35.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Changelog:
> v5: mention the PRs in the commit message. Add tags.
> 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 45a4102e75..df63a92b78 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -350,7 +350,7 @@ 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);
> @@ -371,6 +371,7 @@ 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
>