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Subject[RFC] CPUID usage for interaction between Hypervisors and Linux.

Please find below the proposal for the generic use of cpuid space
allotted for hypervisors. Apart from this cpuid space another thing
worth noting would be that, Intel & AMD reserve the MSRs from 0x40000000
- 0x400000FF for software use. Though the proposal doesn't talk about
MSR's right now, we should be aware of these reservations as we may want
to extend the way we use CPUID to MSR usage as well.

While we are at it, we also think we should form a group which has at
least one person representing each of the hypervisors interested in
generalizing the hypervisor CPUID space for Linux guest OS. This group
will be informed whenever a new CPUID leaf from the generic space is to
be used. This would help avoid any duplicate definitions for a CPUID
semantic by two different hypervisors. I think most of the people are
subscribed to LKML or the virtualization lists and we should use these
lists as a platform to decide on things.



Hypervisor CPUID Interface Proposal

Intel & AMD have reserved cpuid levels 0x40000000 - 0x400000FF for
software use. Hypervisors can use these levels to provide an interface
to pass information from the hypervisor to the guest running inside a
virtual machine.

This proposal defines a standard framework for the way in which the
Linux and hypervisor communities incrementally define this CPUID space.

(This proposal may be adopted by other guest OSes. However, that is not
a requirement because a hypervisor can expose a different CPUID
interface depending on the guest OS type that is specified by the VM

Hypervisor Present Bit:
Bit 31 of ECX of CPUID leaf 0x1.

This bit has been reserved by Intel & AMD for use by
hypervisors, and indicates the presence of a hypervisor.

Virtual CPU's (hypervisors) set this bit to 1 and physical CPU's
(all existing and future cpu's) set this bit to zero. This bit
can be probed by the guest software to detect whether they are
running inside a virtual machine.

Hypervisor CPUID Information Leaf:
Leaf 0x40000000.

This leaf returns the CPUID leaf range supported by the
hypervisor and the hypervisor vendor signature.

# EAX: The maximum input value for CPUID supported by the hypervisor.
# EBX, ECX, EDX: Hypervisor vendor ID signature.

Hypervisor Specific Leaves:
Leaf range 0x40000001 - 0x4000000F.

These cpuid leaves are reserved as hypervisor specific leaves.
The semantics of these 15 leaves depend on the signature read
from the "Hypervisor Information Leaf".

Generic Leaves:
Leaf range 0x40000010 - 0x4000000FF.

The semantics of these leaves are consistent across all
hypervisors. This allows the guest kernel to probe and
interpret these leaves without checking for a hypervisor

A hypervisor can indicate that a leaf or a leaf's field is
unsupported by returning zero when that leaf or field is probed.

To avoid the situation where multiple hypervisors attempt to define the
semantics for the same leaf during development, we can partition
the generic leaf space to allow each hypervisor to define a part
of the generic space.

For instance:
VMware could define 0x4000001X
Xen could define 0x4000002X
KVM could define 0x4000003X
and so on...

Note that hypervisors can implement any leaves that have been
defined in the generic leaf space whenever common features can
be found. For example, VMware hypervisors can implement leafs
that have been defined in the KVM area 0x4000003X and vice

The kernel can detect the support for a generic field inside
leaf 0x400000XY using the following algorithm:

1. Get EAX from Leaf 0x400000000, Hypervisor CPUID information.
EAX returns the maximum input value for the hypervisor CPUID

If EAX < 0x400000XY, then the field is not available.

2. Else, extract the field from the target Leaf 0x400000XY
by doing cpuid(0x400000XY).

If (field == 0), this feature is unsupported/unimplemented
by the hypervisor. The kernel should handle this case
gracefully so that a hypervisor is never required to
support or implement any particular generic leaf.


Definition of the Generic CPUID space.
Leaf 0x40000010, Timing Information.

VMware has defined the first generic leaf to provide timing
information. This leaf returns the current TSC frequency and
current Bus frequency in kHz.

# EAX: (Virtual) TSC frequency in kHz.
# EBX: (Virtual) Bus (local apic timer) frequency in kHz.
# ECX, EDX: RESERVED (Per above, reserved fields are set to zero).


Written By,
Alok N Kataria <>
Dan Hecht <>
Inputs from,
Jun Nakajima <>

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