Features/CPUModels

CPU Models

Summary

This set of features provides a framework allowing cpu model definitions to be configured vs. the existing scheme where such is hard-coded within qemu. The general motivation for this originally was to support contemporary processor architectures directly and intuitively rather than resorting to the use of "-cpu qemu64" augmented with a series of model specific feature flags.

Other considerations were to provide model names reflective of current processors, identify meaningful functional groups within the architecture spectrum to facilitating guest migration, and allowing more accurate and enforceable CPU feature specification by the user.

Owner

• Name: John Cooper
• Email: john.cooper@redhat.com

Current status

• This functionality is available in qemu version 0.13.
• Targeted release: Fedora 13
• Percentage of completion: 100%

Detailed Description

This functionality deprecates the prior hard wired definitions with a configuration file approach for new models. Existing hard-wired models currently remain but are likely to be transitioned to the configuration file representation. At the present they may however be overridden by an identically named model definition in the configuration file.

Proposed new model definitions are provided here for current AMD and Intel processors. Each model consists of a name used to select it on the command line [-cpu <name>], and a model_id which by convention corresponds to a least common denominator commercial instance of the processor class. The following describes how the added CPU model functionality is visible to the command line user.

A table of names/model_ids of all registered CPU definitions may be queried via "-cpu ?model":

       :
   x86       Opteron_G3  AMD Opteron 23xx (Gen 3 Class Opteron)          
   x86       Opteron_G2  AMD Opteron 22xx (Gen 2 Class Opteron)          
   x86       Opteron_G1  AMD Opteron 240 (Gen 1 Class Opteron)           
   x86          Nehalem  Intel Core i7 9xx (Nehalem Class Core i7)       
   x86           Penryn  Intel Core 2 Duo P9xxx (Penryn Class Core 2)    
   x86           Conroe  Intel Celeron_4x0 (Conroe/Merom Class Core 2)
       :         

Also added is "-cpu ?dump" which exhaustively outputs all config data for all defined models, and "-cpu ?cpuid" which enumerates all qemu recognized CPUID feature flags.

The pseudo CPUID flag 'check' when appearing in the command line feature flag list will warn when feature flags (either implicit in a cpu model or explicit on the command line) would have otherwise been quietly unavailable to a guest:

   # qemu-system-x86_64 ... -cpu Nehalem,check
   warning: host cpuid 0000_0001 lacks requested flag 'sse4.2|sse4_2' [0x00100000]
   warning: host cpuid 0000_0001 lacks requested flag 'popcnt' [0x00800000]

A similar 'enforce' pseudo flag exists which in addition to the above causes qemu to error exit if requested flags are unavailable.

Configuration data for a cpu model resides in the target config file which by default will be installed as:

   /usr/local/etc/qemu/target-<arch>.conf

The format of this file should be self explanatory given the definitions for the above six models and essentially mimics the structure of the existing static x86_def_t x86_defs. The CPU model groupings and definitions provided by the default configuration file are believed to be accurate and applicable for the majority of use cases but by definition may be modified to support alternate schemes.

Encoding of CPUID flag names now allows aliases for both the configuration file and the command line which reconciles some Intel/AMD/Linux/Qemu naming differences. An exhaustive dump of CPUID flag names may be obtained via "-cpu ?cpuid".

Benefit to Fedora

Equivalent to benefits for QEMU including intuitive support of contemporary processor models, more accurate and enforceable guest migration classes, extended user interface, and an external CPU model definition mechanism.

Scope

These changes impact the command line user interface including programmatic users of this interface (eg: libvirt).

How To Test

The indicated functionality should be present and operational as described.

User Experience

These changes impact the command line user interface.

Dependencies

This functionality is available beginning with QEMU version 0.13.

Contingency Plan

N/A

Documentation

QEMU CPU Models

Release Notes

As called out above in "Summary".

Comments and Discussion

At the time this documentation was written, a proposed change to the configuration file syntax exists which would cause minor impact to the current structure of the CPU Model configuration file.