virt-manager cannot execute a guest VM with more than 255 vCPU's assigned

This document (000019723) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15 SP2
SUSE Linux Enterprise Server 12 SP5

Situation

It is currently not possible to execute a guest virtual machine which is configured with more than 255 vCPU's on Intel x86 platform.

Resolution

Although there is already general support for >255 vCPU's in the SUSE virtualization stack, it is not necessarily the case that all components of that virtualization stack are also enabled with that functionality.
Currently, virt-manager does not yet have that support available even in the latest upstream version.

This is scheduled to be resolved in a future release of SUSE Linux Enterprise once the current limitations will be resolved in the upstream version of virt-manager.

As a temporary work-around, manually editing the VM's XML file, and modify the details as per the snippets outlined in the diff below :

# diff -c centos7.0-original.xml centos7.0-280-cpu.xml 
*** centos7.0-original.xml	2020-12-04 16:46:42.581626099 +0100
--- centos7.0-280-cpu.xml	2020-12-04 16:47:00.229829521 +0100
***************
*** 15,21 ****
    </metadata>
    <memory unit='KiB'>4194304</memory>
    <currentMemory unit='KiB'>4194304</currentMemory>
!   <vcpu placement='static'>64</vcpu>
    <os>
      <type arch='x86_64' machine='pc-q35-4.2'>hvm</type>
      <boot dev='hd'/>
--- 15,21 ----
    </metadata>
    <memory unit='KiB'>4194304</memory>
    <currentMemory unit='KiB'>4194304</currentMemory>
!   <vcpu placement='static'>280</vcpu>
    <os>
      <type arch='x86_64' machine='pc-q35-4.2'>hvm</type>
      <boot dev='hd'/>
***************
*** 23,28 ****
--- 23,29 ----
    <features>
      <acpi/>
      <apic/>
+     <ioapic driver='qemu'/>
    </features>
    <cpu mode='host-model' check='partial'/>
    <clock offset='utc'>
***************
*** 147,151 ****
--- 148,155 ----
        <backend model='random'>/dev/urandom</backend>
        <address type='pci' domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
      </rng>
+     <iommu model='intel'>
+       <driver intremap='on' eim='on'/>
+     </iommu>
    </devices>
  </domain>

Using an XML file with the setting above, we were able to start a VM, in this sample CentOS, with 280 CPU's assigned

Cause

This is currently a limitation on Intel x86 virtualization stack

Additional Information

The max 288 CPU setting is a hard-coded limit currently.

See also :
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/include/asm/kvm_host.h:

Disclaimer

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