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Revision 57f26ae7

ID57f26ae72983095d0258e391041dfb8864f769e5

Added by Eduardo Habkost over 11 years ago

target-i386: CPUID: return highest basic leaf if eax > cpuid_xlevel

This fixes a subtle bug. A bug that probably won't cause trouble for any
existing OS, but a bug anyway:

Intel SDM Volume 2, CPUID Instruction states:

Two types of information are returned: basic and extended function
information. If a value entered for CPUID.EAX is higher than the maximum
input value for basic or extended function for that processor then the
data for the highest basic information leaf is returned. For example,
using the Intel Core i7 processor, the following is true:

CPUID.EAX = 05H (* Returns MONITOR/MWAIT leaf. )
CPUID.EAX = 0AH ( Returns Architectural Performance Monitoring leaf. )
CPUID.EAX = 0BH ( Returns Extended Topology Enumeration leaf. )
CPUID.EAX = 0CH ( INVALID: Returns the same information as CPUID.EAX = 0BH. )
CPUID.EAX = 80000008H ( Returns linear/physical address size data. )
CPUID.EAX = 8000000AH ( INVALID: Returns same information as CPUID.EAX = 0BH. *)

AMD's CPUID Specification, on the other hand, is less specific:

The CPUID instruction supports two sets or ranges of functions,
standard and extended.

• The smallest function number of the standard function range is
Fn0000_0000. The largest function num- ber of the standard function
range, for a particular implementation, is returned in CPUID
Fn0000_0000_EAX.

• The smallest function number of the extended function range is
Fn8000_0000. The largest function num- ber of the extended function
range, for a particular implementation, is returned in CPUID
Fn8000_0000_EAX.

Functions that are neither standard nor extended are undefined and
should not be relied upon.

QEMU's behavior matched Intel's specification before, but this was
changed by commit b3baa152aaef1905876670590275c2dd0bbb088c. This patch
restores the behavior documented by Intel when cpuid_xlevel2 is 0.

The existing behavior when cpuid_xlevel2 is set (falling back to
level=cpuid_xlevel) is being kept, as I couldn't find any public
documentation on the CPUID 0xC0000000 function range on Centaur CPUs.

Signed-off-by: Eduardo Habkost <>
Signed-off-by: Anthony Liguori <>

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