1 Basics

Status: Supported
Architecture: x86
Component: Hypervisor, toolstack, guest

2 Overview

On native hardware, a kernel will boot, detect features, typically optimise certain codepaths based on the available features, and expect the features to remain available until it shuts down.

The same expectation exists for virtual machines, and it is up to the hypervisor/toolstack to fulfill this expectation for the lifetime of the virtual machine, including across migrate/suspend/resume.

3 User details

Many factors affect the featureset which a VM may use:

A firmware or software upgrade might reduce the available set of features (e.g. Intel disabling TSX in a microcode update for certain Haswell/Broadwell processors), as may editing the settings.

It is unsafe to make any assumption about features remaining consistent across a host reboot. Xen recalculates all information from scratch each boot, and provides the information for the toolstack to consume.

xl currently has no facilities to help the user collect appropriate feature information from relevant hosts and compute appropriate feature specifications for use in host or domain configurations. (xl being a single-host toolstack, it would in any case need external support for accessing remote hosts eg via ssh, in the form of automation software like GNU parallel or ansible.)

4 Technical details

The CPUID instruction is used by software to query for features. In the virtualisation usecase, guest software should query Xen rather than hardware directly. However, CPUID is an unprivileged instruction which doesn't fault, complicating the task of hiding hardware features from guests.

Important files:

4.1 Ability to control CPUID

4.1.1 HVM

HVM guests (using Intel VT-x or AMD SVM) will unconditionally exit to Xen on all CPUID instructions, allowing Xen full control over all information.

4.1.2 PV

The CPUID instruction is unprivileged, so executing it in a PV guest will not trap, leaving Xen no direct ability to control the information returned.

4.1.3 Xen Forced Emulation Prefix

Xen-aware PV software can make use of the 'Forced Emulation Prefix'

ud2a; .ascii 'xen'; cpuid

which Xen recognises as a deliberate attempt to get the fully-controlled CPUID information rather than the hardware-reported information. This only works with cooperative software.

4.1.4 Masking and Override MSRs

AMD CPUs from the K8 onwards support Feature Override MSRs, which allow direct control of the values returned for certain CPUID leaves. These MSRs allow any result to be returned, including the ability to advertise features which are not actually supported.

Intel CPUs between Nehalem and SandyBridge have differing numbers of Feature Mask MSRs, which are a simple AND-mask applied to all CPUID instructions requesting specific feature bitmap sets. The exact MSRs, and which feature bitmap sets they affect are hardware specific. These MSRs allow features to be hidden by clearing the appropriate bit in the mask, but does not allow unsupported features to be advertised.

4.1.5 CPUID Faulting

Intel CPUs from IvyBridge onwards have CPUID Faulting, which allows Xen to cause CPUID instruction executed in PV guests to fault. This allows Xen full control over all information, exactly like HVM guests.

4.2 Compile time

As some features depend on other features, it is important that, when disabling a certain feature, we disable all features which depend on it. This allows runtime logic to be simplified, by being able to rely on testing only the single appropriate feature, rather than the entire feature dependency chain.

To speed up runtime calculation of feature dependencies, the dependency chain is calculated and flattened by xen/tools/gen-cpuid.py to create xen/include/asm-x86/cpuid-autogen.h from xen/include/public/arch-x86/cpufeatureset.h, allowing the runtime code to disable all dependent features of a specific disabled feature in constant time.

4.3 Host boot

As Xen boots, it will enumerate the features it can see. This is stored as the raw_featureset.

Errata checks and command line arguments are then taken into account to reduce the raw_featureset into the host_featureset, which is the set of features Xen uses. On hardware with masking/override MSRs, the default MSR values are picked from the host_featureset.

The host_featureset is then used to calculate the pv_featureset and hvm_featureset, which are the maximum featuresets Xen is willing to offer to PV and HVM guests respectively.

In addition, Xen will calculate how much control it has over non-cooperative PV CPUID instructions, storing this information as levelling_caps.

4.4 Domain creation

The toolstack can query each of the calculated featureset via XEN_SYSCTL_get_cpu_featureset, and query for the levelling caps via XEN_SYSCTL_get_cpu_levelling_caps.

These data should be used by the toolstack when choosing the eventual featureset to offer to the guest.

Once a featureset has been chosen, it is set (implicitly or explicitly) via XEN_DOMCTL_set_cpuid. Xen will clamp the toolstacks choice to the appropriate PV or HVM featureset. On hardware with masking/override MSRs, the guest cpuid policy is reflected in the MSRs, which are context switched with other vcpu state.

5 Limitations

A guest which ignores the provided feature information and manually probes for features will be able to find some of them. e.g. There is no way of forcibly preventing a guest from using 1GB superpages if the hardware supports it.

Some information simply cannot be hidden from guests. There is no way to control certain behaviour such as the hardware MXCSR_MASK or x87 FPU exception behaviour.

6 Testing

Feature levelling is a very wide area, and used all over the hypervisor. Please ask on xen-devel for help identifying more specific tests which could be of use.

7 Known issues / Areas for improvement

The feature querying and levelling functions should exposed in a convenient-to-use way by xl.

Xen currently has no concept of per-{socket,core,thread} CPUID information. As a result, details such as APIC IDs, topology and cache information do not match real hardware, and do not match the documented expectations in the Intel and AMD system manuals.

The CPU feature flags are the only information which the toolstack has a sensible interface for querying and levelling. Other information in the CPUID policy is important and should be levelled (e.g. maxphysaddr).

The CPUID policy is currently regenerated from scratch by the receiving side, once memory and vcpu content has been restored. This means that the receiving Xen cannot verify the memory/vcpu content against the CPUID policy, and can end up running a guest which will subsequently crash. The CPUID policy should be at the head of the migration stream.

MSRs are another source of features for guests. There is no general provision for controlling the available MSRs. E.g. 64bit versions of Windows notice changes in IA32_MISC_ENABLE, and suffer a BSOD 0x109 (Critical Structure Corruption)

8 References

Intel Flexmigration

AMD Extended Migration Technology

9 History

Date Revision Version Notes
2016-05-31 1 Xen 4.7 Document written