VM Panic / Crash During or After Startup Due to Guest OS Attempting to Modify PAM Registers in VM BIOS

Products

• VMware vSphere ESXi

Issue/Introduction

A virtual machine running Red Hat Enterprise Linux (RHEL) 7 experiences an unexpected crash and reboot approximately 1 minute after power-on (or cold start/vMotion recovery). The VMX process panics with a VERIFY failure in the chipset emulation code:

PANIC: VERIFY bora/devices/misc/x86/cs440bx.c:1094

From vmfs/volumes/DatastoreUUID/VMname/vmware.log excerpts (multiple occurrences observed on the same VM across restarts):

yyyy-mm-ddThh:mm:ss.###Z Cr(01) vcpu-0 - PANIC: VERIFY bora/devices/misc/x86/cs440bx.c:1094
yyyy-mm-ddThh:mm:ss.###Z Cr(01) vcpu-1 - PANIC: VERIFY bora/devices/misc/x86/cs440bx.c:1094
yyyy-mm-ddThh:mm:ss.###Z Cr(01) vcpu-0 - PANIC: VERIFY bora/devices/misc/x86/cs440bx.c:1094
yyyy-mm-ddThh:mm:ss.###Z Cr(01) vcpu-0 - PANIC: VERIFY bora/devices/misc/x86/cs440bx.c:1094

Common symptoms and signatures include:

• Abrupt virtual machine shut down or HA-initiated restart.

• Lack of guest-side kernel panic dumps or OS system logs preceding the crash event.

• Structural failures relating to legacy BIOS or Pluggable Authentication Modules (PAM) emulation.

Environment

VMware vSphere ESXi 8.x

RHEL 7.x

Cause

The panic occurs in i440BX chipset emulation when the guest OS (RHEL 7) or BIOS attempts a PAM register write to change legacy memory attributes (0xc0000 range) to write-only DRAM , invalid in virtual BIOS emulation (VERIFY prevents corruption).

Resolution

1. Permanent Prevention (Strongly Recommended)

Switch the virtual machine's boot firmware from legacy BIOS to EFI/UEFI to bypass the legacy emulation layers entirely:

1. Power off the virtual machine.

2. Right-click the VM and select Edit Settings.

3. Navigate to the VM Options tab and expand Boot Options.

4. Change the Firmware drop-down menu from BIOS to EFI.

5. Click Save and power on the virtual machine.

Note: Verify that the guest operating system is fully compatible with UEFI boot modes before executing this conversion.

2. Workarounds (If memory or vMotion overhead pressure is suspected)

If firmware migration cannot be performed immediately, apply one of the following resource adjustments:

• Configure VM Overhead Growth Limit:

  1. Power off the virtual machine.

  2. Append the following advanced parameter to the VM's .vmx configuration file:

     VMOverheadGrowthLimit = "268435456"
     

     (This sets the memory growth ceiling to 256 MB; increase this value proportionately based on memory workload).

  3. Power on the virtual machine.

• Configure Memory Reservation: Apply a dedicated physical memory reservation of at least +512 MB to 1 GB on the virtual machine settings via the vSphere Client to guarantee necessary overhead blocks during high-stress operations.