[PATCH v2] pc: q35: Bump max_cpus to 1856 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 1856 vcpus and no more both with edk2 and seabios firmwares.
If an additional vcpu is added, that is with 1857 vcpus, edk2 currently fails
with the following error messages:

AllocatePages failed: No 0x400 Pages is available.
There is only left 0x2BF pages memory resource to be allocated.
ERROR: Out of aligned pages
ASSERT /builddir/build/BUILD/edk2-ba91d0292e/MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.c(814): BigPageAddress != 0

This error exists only with edk2. Seabios currently can boot a linux guest
fine with 4096 vcpus. Since the lowest common denominator for a working VM for
both edk2 and seabios is 1856 vcpus, bump up the value max_cpus to 1856 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:
v2: bump up the vcpu number to 1856. Add failure messages from ekd2 in
the commit description.
See also RH Jira https://issues.redhat.com/browse/RHEL-22202

diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index f43d5142b8..f9c4b6594d 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 = 1856;
     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