Feature and Interface Discovery

Article
07/08/2022

Guest software interacts with the hypervisor through a variety of mechanisms. Many of these mirror the traditional mechanisms used by software to interact with the underlying processor. As such, these mechanisms are architecture-specific. On the x64 architecture, the following mechanisms are used:

CPUID instruction – Used for static feature and version information.
MSRs (model-specific registers) – Used for status and control values.
Memory-mapped registers – Used for status and control values.
Processor interrupts – Used for asynchronous events, notifications and messages.

In addition to these architecture-specific interfaces, the hypervisor provides a simple procedural interface implemented with hypercalls.

Hypervisor Discovery

Before using any hypervisor interfaces, software should first determine whether it’s running within a virtualized environment. On x64 platforms that conform to this specification, this is done by executing the CPUID instruction with an input (EAX) value of 1. Upon execution, code should check bit 31 of register ECX (the “hypervisor present bit”). If this bit is set, a hypervisor is present. In a non-virtualized environment, the bit will be clear.

CPUID.01h.ECX:31 // if set, virtualization present

If the “hypervisor present bit” is set, additional CPUID leafs can be queried for more information about the conformant hypervisor and its capabilities. Two such leaves are guaranteed to be available: 0x40000000 and 0x40000001. Subsequently-numbered leaves may also be available.

Standard Hypervisor CPUID Leaves

When the leaf at 0x40000000 is queried, the hypervisor will return information that provides the maximum hypervisor CPUID leaf number and a vendor ID signature.

Register	Information Provided
EAX	The maximum input value for hypervisor CPUID information
EBX	Hypervisor Vendor ID Signature
ECX	Hypervisor Vendor ID Signature
EDX	Hypervisor Vendor ID Signature

If the leaf at 0x40000001 is queried, it will return a value representing a vendor-neutral hypervisor interface identification. This determines the semantics of the leaves from 0x4000002 through 0x400000FF.

Register	Information Provided
EAX	Hypervisor Interface Signature.
EBX	Reserved
ECX	Reserved
EDX	Reserved

These two leaves allow the guest to query the hypervisor vendor ID and interface independently. The vendor ID is provided only for informational and diagnostic purposes. It is recommended that software only base compatibility decisions on the interface signature reported through leaf 0x40000001.

Microsoft Hypervisor CPUID Leaves

On hypervisors conforming to the Microsoft hypervisor CPUID interface, the 0x40000000 and 0x40000001 leaf registers will have the following values.

Hypervisor CPUID Leaf Range - 0x40000000

EAX determines the maximum hypervisor CPUID leaf. EBX-EDX contain the hypervisor vendor ID signature. The vendor ID signature should be used only for reporting and diagnostic purposes.

Register	Information Provided
EAX	The maximum input value for hypervisor CPUID information. On Microsoft hypervisors, this will be at least `0x40000005`.
EBX	0x7263694D—“Micr”
ECX	0x666F736F—“osof”
EDX	0x76482074—“t Hv”

Hypervisor Vendor-Neutral Interface Identification - 0x40000001

EAX contains the hypervisor interface identification signature. This determines the semantics of the leaves from 0x40000002 through 0x400000FF.

Register	Information Provided
EAX	0x31237648—“Hv#1”
EBX	Reserved
ECX	Reserved
EDX	Reserved

Hypervisors conforming to the “Hv#1” interface also provide at least the following leaves.

Hypervisor System Identity - 0x40000002

Register	Bits	Information Provided
EAX		Build Number
EBX	31-16	Major Version
EBX	15-0	Minor Version

Hypervisor Feature Identification - 0x40000003

EAX and EBX indicate which features are available to the partition based upon the current partition privileges.

Register	Bits	Information Provided
EAX		Corresponds to bits 31-0 of HV_PARTITION_PRIVILEGE_MASK
EBX		Corresponds to bits 63-32 of HV_PARTITION_PRIVILEGE_MASK
ECX	4-0	Reserved
	5	Invariant Mperf is available
	6	Supervisor shadow stack is available
	7	Architectural PMU is available
	8	Exception trap intercept is available
	31-9	Reserved
EDX	0	Deprecated (previously indicated availability of the MWAIT instruction)
	1	Guest debugging support is available
	2	Performance Monitor support is available
	3	Support for physical CPU dynamic partitioning events is available
	4	Support for passing hypercall input parameter block via XMM registers is available
	5	Support for a virtual guest idle state is available
	6	Support for hypervisor sleep state is available
	7	Support for querying NUMA distances is available
	8	Support for determining timer frequencies is available
	9	Support for injecting synthetic machine checks is available
	10	Support for guest crash MSRs is available
	11	Support for debug MSRs is available
	12	Support for NPIEP is available
	13	DisableHypervisorAvailable
	14	ExtendedGvaRangesForFlushVirtualAddressListAvailable
	15	Support for returning hypercall output via XMM registers is available
	16	Reserved
	17	SintPollingModeAvailable
	18	HypercallMsrLockAvailable
	19	Use direct synthetic timers
	20	Support for PAT register available for VSM
	21	Support for bndcfgs register available for VSM
	22	Reserved
	23	Support for synthetic time unhalted timer available
	25-24	Reserved
	26	Intel’s Last Branch Record (LBR) feature supported
	31-27	Reserved

Implementation Recommendations - 0x40000004

Indicates which behaviors the hypervisor recommends the OS implement for optimal performance.

Register	Bits	Information Provided
EAX	0	Recommend using hypercall for address space switches rather than MOV to CR3 instruction.
	1	Recommend using hypercall for local TLB flushes rather than INVLPG or MOV to CR3 instructions.
	2	Recommend using hypercall for remote TLB flushes rather than inter-processor interrupts.
	3	Recommend using MSRs for accessing APIC registers EOI, ICR and TPR rather than their memory-mapped counterparts.
	4	Recommend using the hypervisor-provided MSR to initiate a system RESET.
	5	Recommend using relaxed timing for this partition. If used, the VM should disable any watchdog timeouts that rely on the timely delivery of external interrupts.
	6	Recommend using DMA remapping.
	7	Recommend using interrupt remapping.
	8	Reserved.
	9	Recommend deprecating AutoEOI.
	10	Recommend using SyntheticClusterIpi hypercall.
	11	Recommend using the newer ExProcessorMasks interface.
	12	Indicates that the hypervisor is nested within a Hyper-V partition.
	13	Recommend using INT for MBEC system calls.
	14	Recommend a nested hypervisor using the enlightened VMCS interface. Also indicates that additional nested enlightenments may be available (see leaf 0x4000000A).
	15	UseSyncedTimeline – Indicates the partition should consume the QueryPerformanceCounter bias provided by the root partition.
	16	Reserved
	17	UseDirectLocalFlushEntire – Indicates the guest should toggle CR4.PGE to flush the entire TLB, as this is more performant than making a hypercall.
	18	NoNonArchitecturalCoreSharing - indicates that core sharing is not possible. This can be used as an optimization to avoid the performance overhead of STIBP.
	31-19	Reserved
EBX		Recommended number of attempts to retry a spinlock failure before notifying the hypervisor about the failures. 0xFFFFFFFF indicates never notify.
ECX	6-0	ImplementedPhysicalAddressBits – Reports the physical address width (MAXPHYADDR) reported by the system’s physical processors. If all bits contain 0, the feature is not supported. Note that the value reported is the actual number of physical address bits, and not the bit position used to represent that number.
ECX	31-7	Reserved
EDX		Reserved

Hypervisor Implementation Limits - 0x40000005

Describes the scale limits supported in the current hypervisor implementation. If any value is zero, the hypervisor does not expose the corresponding information; otherwise, they have these meanings.

Register	Information Provided
EAX	The maximum number of virtual processors supported
EBX	The maximum number of logical processors supported
ECX	The maximum number of physical interrupt vectors available for interrupt remapping.
EDX	Reserved

Implementation Hardware Features - 0x40000006

Indicates which hardware-specific features have been detected and are currently in use by the hypervisor.

Register	Bits	Information Provided
EAX	0	Support for APIC overlay assist is detected and in use.
	1	Support for MSR bitmaps is detected and in use.
	2	Support for architectural performance counters is detected and in use.
	3	Support for second level address translation is detected and in use.
	4	Support for DMA remapping is detected and in use.
	5	Support for interrupt remapping is detected and in use.
	6	Indicates that a memory patrol scrubber is present in the hardware.
	7	DMA protection is in use.
	8	HPET is requested.
	9	Synthetic timers are volatile.
	13-10	The hypervisor level of the current guest - '0' if non-nested.
	14	Physical destination mode required.
	15	Reserved
	16	Support for hardware memory zeroing is present.
	17	Support for Unrestricted Guest is present.
	18	Support for resource allocation (RDT-A, PQOS-A) is present.
	19	Support for resource monitoring (RDT-M, PQOS-M) is present.
	20	Support for guest virtual PMU is present.
	21	Support for guest virtual LBR is present.
	22	Support for guest virtual IPT is present.
	23	Support for APIC emulation is present.
	24	ACPI WDAT table is detected and in use by the hypervisor.
	31-25	Reserved
EBX		Reserved
ECX		Reserved
EDX		Reserved

Nested Hypervisor Feature Identification - 0x40000009

Describes the features exposed to the partition by the hypervisor when running nested. EAX describes access to virtual MSRs. EDX describes access to hypercalls.

Register	Bits	Information Provided
EAX	1-0	Reserved
	2	AccessSynicRegs
	3	Reserved
	4	AccessIntrCtrlRegs
	5	AccessHypercallMsrs
	6	AccessVpIndex
	11-7	Reserved
	12	AccessReenlightenmentControls
	31-13	Reserved
EBX		Reserved
ECX		Reserved
EDX	3-0	Reserved
	4	XmmRegistersForFastHypercallAvailable
	14-5	Reserved
	15	FastHypercallOutputAvailable
	16	Reserved
	17	SintPollingModeAvailable
	31-18	Reserved

Hypervisor Nested Virtualization Features - 0x4000000A

Indicates which nested virtualization optimizations are available to a nested hypervisor.

Register	Bits	Information Provided
EAX	7-0	Enlightened VMCS version (low)
	15-8	Enlightened VMCS version (high)
	16	Reserved
	17	Indicates support for direct virtual flush hypercalls.
	18	Indicates support for the HvFlushGuestPhysicalAddressSpace and HvFlushGuestPhysicalAddressList hypercalls (on Intel platforms).
	19	Indicates support for using an enlightened MSR bitmap.
	20	Indicates support for combining virtualization exceptions in the page fault exception class.
	21	Indicates support for non-zero value of the 0x00002802 (GuestIa32DebugCtl) field in the VMCS.
	22	Indicates support for the enlightened TLB on AMD platforms. ASID flushes do not affect TLB entries derived from the NPT. Hypercalls must be used to invalidate NPT TLB entries. Also indicates support for the HvFlushGuestPhysicalAddressSpace and HvFlushGuestPhysicalAddressList hypercalls.
	31-21	Reserved
EBX	0	Indicates support for the GuestPerfGlobalCtrl and HostPerfGlobalCtrl fields in the enlightened VMCS.
EBX	31-1	Reserved
ECX		Reserved
EDX		Reserved

Versioning

The hypervisor version information is encoded in leaf 0x40000002. Two version numbers are provided: the main version and the service version.

The main version includes a major and minor version number and a build number. These correspond to Microsoft Windows release numbers. The service version describes changes made to the main version.

Clients are strongly encouraged to check for hypervisor features by using CPUID leaves 0x40000003 through 0x40000005 rather than by comparing against version ranges.

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