xcall(9) - NetBSD Manual Pages

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XCALL(9)               NetBSD 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 ALSO

     kpreempt(9), percpu(9)


HISTORY

     The xcall interface first appeared in NetBSD 5.0.


AUTHORS

     Andrew Doran <ad@NetBSD.org>

NetBSD 6.0                     October 24, 2011                     NetBSD 6.0