XCALL(9) BSD Kernel Developer's Manual XCALL(9)NAME
xcall, xc_broadcast, xc_unicast, xc_wait — cross-call interface
SYNOPSIS
#include <sys/xcall.h>
typedef void (*xcfunc_t)(void *, void *);
uint64_t
xc_broadcast(u_int flags, xcfunc_t func, void *arg1, void *arg2);
uint64_t
xc_unicast(u_int flags, xcfunc_t func, void *arg1, void *arg2,
struct cpu_info *ci);
void
xc_wait(uint64_t where);
DESCRIPTION
The machine-independent xcall interface allows any CPU in the system to
request that an arbitrary function be executed on any other CPU.
Sometimes it is necessary to modify hardware state that is tied directly
to individual CPUs (such as a CPU's local timer), and these updates can
not be done remotely by another CPU. The LWP requesting the update may
be unable to guarantee that it will be running on the CPU where the
update must occur, when the update occurs.
Additionally, it is sometimes necessary to modify per-CPU software state
from a remote CPU. Where these update operations are so rare or the
access to the per-CPU data so frequent that the cost of using locking or
atomic operations to provide coherency is prohibitive, another way must
be found.
Cross calls help to solve these types of problem. However, since this
facility is heavyweight, it is expected that it will not be used often.
xcall provides a mechanism for making “low priority” cross calls. The
function to be executed runs on the remote CPU within a thread context,
and not from a software interrupt, so it can ensure that it is not inter‐
rupting other code running on the CPU, and so has exclusive access to the
CPU. Keep in mind that unless disabled, it may cause a kernel preemp‐
tion.
xcall also provides a mechanism for making “high priority” cross calls.
The function to be executed runs on the remote CPU within a IPL_SOFTCLOCK
software interrupt context, possibly interrupting other lower-priority
code running on the CPU.
NOTES
Functions being called should be relatively lightweight. They may block
on locks, but carefully and minimally, to not interfere with other cross
calls in the system.
FUNCTIONS
xc_broadcast(flags, func, arg1, arg2)
Call (*func)(arg1, arg2) on all CPUs in the system. Return a
uint64_t “ticket” to xc_wait() on for the cross-call to com‐
plete. flags should be XC_HIGHPRI for a "high priority" call,
and 0 for a "low priority" call. xc_broadcast() should not be
called from interrupt context.
xc_unicast(flags, func, arg1, arg2, ci)
Like xc_broadcast(), but call (*func)() on only the CPU indi‐
cated by ci. xc_unicast() also returns a “ticket”.
xc_wait(where)
Wait on the “ticket” returned by a prior xc_broadcast() or
xc_unicast() for the corresponding cross-call to complete.
xc_wait() should be called from a thread context.
CODE REFERENCES
The xcall interface is implemented within the file sys/kern/subr_xcall.c.
SEE ALSOkpreempt(9), percpu(9)HISTORY
The xcall interface first appeared in NetBSD 5.0.
AUTHORS
Andrew Doran ⟨ad@NetBSD.org⟩
BSD October 24, 2011 BSD
