[PATCH] pc: q35: Bump max_cpus to 1728 vcpus

[Ani Sinha]

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. QEMU has been tested to correctly boot a linux guest
with 1728 vcpus both with edk2 and seabios firmwares. So bump up the max_cpus
value for q35 machines versions 9 and newer to 1728. Q35 machines versions
8.2 and older continue to support 1024 maximum vcpus as before for
compatibility.

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>
Signed-off-by: Ani Sinha <anisinha@redhat.com>
---
 hw/i386/pc_q35.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index f43d5142b8..bfa627a70b 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 = 1728;
     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);
@@ -396,6 +396,7 @@ 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