IA32VMXPROCBASEDCTLS

IA32_VMX_PROC_BASED_CTLS, often referred to by the shorthand IA32_VMX_PROC_BASED_CTLS, is a model-specific register (MSR) used in Intel VT-x virtualization. It is read through the processor’s MSR interface and reports the processor’s capabilities for the primary processor-based VM-execution controls in VMX operation. The MSR is typically located at address 0x481 and provides the bitmask that defines which primary VM-Execution controls can be used when configuring a VMCS.

The value of IA32_VMX_PROC_BASED_CTLS is a 64-bit bitfield. It encodes which features and controls in the primary processor-based VM-execution domain are supported by the running CPU. When a hypervisor or VMM prepares a VMCS, it must ensure that the requested settings for VM-entry and VM-exit controls are a subset of the bits allowed by this MSR. Attempting to enable a control that is not supported by the processor can cause VM-entry or VM-exit failures and may trigger a VM VMX violation.

Some processors also expose related capability MSRs and, on newer generations, “true” forms of these capability MSRs that reflect the hardware’s actual supported bits. In practice, IA32_VMX_PROC_BASED_CTLS is used in conjunction with VMX_BASIC and other VMX capability MSRs to determine the appropriate configuration for VMCS fields and to ensure compatibility across CPU models and microarchitectures.