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.Dd March 15, 2017
.Dt KVM_GETPCPU 3
.Os
.Sh NAME
.Nm kvm_dpcpu_setcpu ,
.Nm kvm_getmaxcpu ,
.Nm kvm_getpcpu
.Nd access per-CPU data
.Sh LIBRARY
.Lb libkvm
.Sh SYNOPSIS
.In sys/param.h
.In sys/pcpu.h
.In sys/sysctl.h
.In kvm.h
.Ft int
.Fn kvm_dpcpu_setcpu "kvm_t *kd" "u_int cpu"
.Ft int
.Fn kvm_getmaxcpu "kvm_t *kd"
.Ft int
.Fn kvm_getncpus "kvm_t *kd"
.Ft void *
.Fn kvm_getpcpu "kvm_t *kd" "int cpu"
.Ft ssize_t
.Fn kvm_read_zpcpu "kvm_t *kd" "u_long base" "void *buf" "size_t size" "int cpu"
.Ft uint64_t
.Fn kvm_counter_u64_fetch "kvm_t *kd" "u_long base"
.Sh DESCRIPTION
The
.Fn kvm_dpcpu_setcpu ,
.Fn kvm_getmaxcpu ,
and
.Fn kvm_getpcpu
functions are used to access the per-CPU data of active processors in the
kernel indicated by
.Fa kd .
Per-CPU storage comes in two flavours: data stored directly in a
.Vt "struct pcpu"
associated with each CPU, and dynamic per-CPU storage (DPCPU), in which a
single kernel symbol refers to different data depending on what CPU it is
accessed from.
.Pp
The
.Fn kvm_getmaxcpu
function returns the maximum number of CPUs supported by the kernel.
.Pp
The
.Fn kvm_getncpus
function returns the current number of CPUs in the kernel.
.Pp
The
.Fn kvm_getpcpu
function returns a buffer holding the per-CPU data for a single CPU.
This buffer is described by the
.Vt "struct pcpu"
type.
The caller is responsible for releasing the buffer via a call to
.Xr free 3
when it is no longer needed.
If
.Fa cpu
is not active, then
.Dv NULL
is returned instead.
.Pp
The
.Fn kvm_read_zpcpu
function is used to obtain private per-CPU copy from a
.Dv UMA_ZONE_PCPU
.Xr zone 9 .
It takes
.Fa base
argument as base address of an allocation and copyies
.Fa size
bytes into
.Fa buf
from the part of allocation that is private to
.Fa cpu .
.Pp
The
.Fn kvm_counter_u64_fetch
function fetches value of a
.Xr counter 9
pointed by
.Fa base
address.
.Pp
Symbols for dynamic per-CPU data are accessed via
.Xr kvm_nlist 3
as with other symbols.
.Nm libkvm
maintains a notion of the "current CPU", set by
.Fn kvm_dpcpu_setcpu ,
which defaults to 0.
Once another CPU is selected,
.Xr kvm_nlist 3
will return pointers to that data on the appropriate CPU.
.Sh CACHING
.Fn kvm_getmaxcpu
and
.Fn kvm_getpcpu
cache the nlist values for various kernel variables which are
reused in successive calls.
You may call either function with
.Fa kd
set to
.Dv NULL
to clear this cache.
.Sh RETURN VALUES
On success, the
.Fn kvm_getmaxcpu
function returns the maximum number of CPUs supported by the kernel.
If an error occurs,
it returns -1 instead.
.Pp
On success, the
.Fn kvm_getpcpu
function returns a pointer to an allocated buffer or
.Dv NULL .
If an error occurs,
it returns -1 instead.
.Pp
On success, the
.Fn kvm_dpcpu_setcpu
call returns 0; if an error occurs, it returns -1 instead.
.Pp
On success, the
.Fn kvm_read_zpcpu
function returns number of bytes copied.
If an error occurs, it returns -1 instead.
.Pp
If any function encounters an error,
then an error message may be retrieved via
.Xr kvm_geterr 3 .
.Sh SEE ALSO
.Xr free 3 ,
.Xr kvm 3 ,
.Xr counter 9 ,
.Xr zone 9
