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

[Ani Sinha <anisinha@redhat.com>]

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>
---
 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