CPUID: Difference between revisions
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Some CPUID attributes require multiple CPUID calls to retrieve (often EAX is first set to 0 to determine the highest attribute request supported, then called again with specific requests). Behavior is undefined if CPUID information from one processor is used to form requests for another (especially in a heterogeneous system). Furthermore, CPUID must be performed on each processor used -- results for one processor have no general ramifications for others (again, especially in a heterogeneous system). Thus, either: | Some CPUID attributes require multiple CPUID calls to retrieve (often EAX is first set to 0 to determine the highest attribute request supported, then called again with specific requests). Behavior is undefined if CPUID information from one processor is used to form requests for another (especially in a heterogeneous system). Furthermore, CPUID must be performed on each processor used -- results for one processor have no general ramifications for others (again, especially in a heterogeneous system). Thus, either: | ||
* the process using CPUID must be bound to a single CPU by the system administrator, | * the process using CPUID must be bound to a single CPU by the system administrator, | ||
* the process must be able to bind itself to a CPU ( | * the process must be able to bind itself to a CPU (see <tt>sched_setpolicy()</tt> on [[Linux APIs|Linux]] and <tt>cpuset_setpolicy</tt> on [[FreeBSD APIs|FreeBSD]]), or | ||
* the operating system must provide a world-readable interface (ie, the process never calls CPUID directly) | * the operating system must provide a world-readable interface (ie, the process never calls CPUID directly) | ||
On [[Linux APIs|Linux]], the <tt>/dev/cpuid(4)</tt> device provides an obsolete interface to CPUID. On modern systems, it is chmod'd 0440, and its use is discouraged. Instead, since [http://lkml.indiana.edu/hypermail/linux/kernel/0801.3/2419.html at least 2.6.24] a [[sysfs]] interface has been supported; see <tt>/sys/devices/system/cpu/*/</tt>:<pre>[dumbledore](0) $ for i in /sys/devices/system/cpu/cpu0/cache/*/* /sys/devices/system/cpu/cpu0/topology/* ; do echo -n "`basename $i`: " && cat $i ; done | On [[Linux APIs|Linux]], the <tt>/dev/cpuid(4)</tt> device provides an obsolete interface to CPUID. On modern systems, it is chmod'd 0440, and its use is discouraged. Instead, since [http://lkml.indiana.edu/hypermail/linux/kernel/0801.3/2419.html at least 2.6.24] a [[sysfs]] interface has been supported; see <tt>/sys/devices/system/cpu/*/</tt>:<pre>[dumbledore](0) $ for i in /sys/devices/system/cpu/cpu0/cache/*/* /sys/devices/system/cpu/cpu0/topology/* ; do echo -n "`basename $i`: " && cat $i ; done | ||