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Man Pages
CPUCTL(4) FreeBSD Kernel Interfaces Manual CPUCTL(4)

cpuctl
cpuctl pseudo device

To compile this driver into the kernel, place the following lines in your kernel configuration file:
device cpuctl

Alternatively, to load the driver as a module at boot time, place the following in loader.conf(5):

cpuctl_load="YES"

The special device /dev/cpuctl presents interface to the system CPU. It provides functionality to retrieve CPUID information, read/write machine specific registers (MSR) and perform CPU firmware updates.

For each CPU present in the system, the special device /dev/cpuctl%d with the appropriate index will be created. For multicore CPUs such a special device will be created for each core.

Currently, only i386 and amd64 processors are supported.

All of the supported operations are invoked using the ioctl(2) system call. Currently, the following ioctls are defined:
cpuctl_msr_args_t *args
 
cpuctl_msr_args_t *args
Read/write CPU machine specific register. The cpuctl_msr_args_t structure is defined in <sys/cpuctl.h> as:
typedef struct {
	int		msr;	/* MSR to read */
	uint64_t	data;
} cpuctl_msr_args_t;
    
cpuctl_msr_args_t *args
 
cpuctl_msr_args_t *args
Set/clear MSR bits according to the mask given in the data field.
cpuctl_cpuid_args_t *args
Retrieve CPUID information. Arguments are supplied in the following structure:
typedef struct {
	int		level;		/* CPUID level */
	uint32_t	data[4];
} cpuctl_cpuid_args_t;
    
It is equivalent to the CPUCTL_CPUID_COUNT request with level_type set to 0.
cpuctl_cpuid_count_args_t *args
Retrieve CPUID information. Arguments are supplied in the following structure:
typedef struct {
	int		level;		/* CPUID level */
	int		level_type;	/* CPUID level type */
	uint32_t	data[4];
} cpuctl_cpuid_count_args_t;
    
The level field indicates the CPUID level to retrieve, it is loaded into the %eax register before the CPUID instruction is executed, The level_type field indicates the CPUID level type to retrieve, it is loaded into the %ecx register.

The data field is used to store the received CPUID data. That is, data[0] contains the value of %eax register after the CPUID instruction is executed, data[1] is for %ebx, data[2] for %ecx, and data[3] for %edx.

Update CPU firmware (microcode). The structure is defined in <sys/cpuctl.h> as:
typedef struct {
	void	*data;
	size_t	size;
} cpuctl_update_args_t;
    

The data field should point to the firmware image of size size.

For additional information refer to cpuctl.h.

/dev/cpuctl
 

[]
The operation requested is not supported by the device (e.g., unsupported architecture or the CPU is disabled).
[]
Incorrect request was supplied, or microcode image is not correct.
[]
No physical memory was available to complete the request.
[]
The firmware image address points outside the process address space.

hwpmc(4), cpucontrol(8)

The cpuctl driver first appeared in FreeBSD 7.2.

The cpuctl module and this manual page were written by Stanislav Sedov <stas@FreeBSD.org>.

Yes, probably, report if any.
June 20, 2014 FreeBSD 13.1-RELEASE

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