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>
---
 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);
 }
-- 
2.42.0

On Wed, Feb 21, 2024 at 07:32:27PM +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 with edk2 pending various upstream EDK2 fixes
> which will probably be in the 2024-05 release to be released in the coming

Merged meanwhile, so 2024-05 release is a sure thing and latest edk2
master branch is good too.

You might refine the commit message saying so.
With or without that:
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>

take care,
  Gerd

> On 28-Feb-2024, at 17:12, Gerd Hoffmann <kraxel@redhat.com> wrote:
> 
> On Wed, Feb 21, 2024 at 07:32:27PM +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 with edk2 pending various upstream EDK2 fixes
>> which will probably be in the 2024-05 release to be released in the coming
> 
> Merged meanwhile, so 2024-05 release is a sure thing and latest edk2
> master branch is good too.
> 
> You might refine the commit message saying so.

Can I refer to your two pull requests 
https://github.com/tianocore/edk2/pull/5410
https://github.com/tianocore/edk2/pull/5418

In the commit log? I think they are not ephemeral.

> With or without that:
> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> take care,
>  Gerd
> 

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);
>  }