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XCALL(9)                   Kernel Developer's Manual                  XCALL(9)

     xcall, xc_broadcast, xc_unicast, xc_wait - cross-call interface

     #include <sys/xcall.h>

     typedef void (*xcfunc_t)(void *, void *);

     xc_broadcast(u_int flags, xcfunc_t func, void *arg1, void *arg2);

     xc_unicast(u_int flags, xcfunc_t func, void *arg1, void *arg2,
         struct cpu_info *ci);

     xc_wait(uint64_t where);

     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
     interrupting 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

     xcall also provides a mechanism for making "high priority" cross calls.
     The function to be executed runs on the remote CPU within a software
     interrupt context, possibly interrupting other lower-priority code
     running on the CPU.

     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.

     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
              complete.  flags should be XC_HIGHPRI or XC_HIGHPRI_IPL(ipl) for
              a "high priority" call, and 0 for a "low priority" call.
              XC_HIGHPRI uses an IPL_SOFTSERIAL software interrupt while
              XC_HIGHPRI_IPL() uses a software interrupt with an IPL specified
              by ipl.  xc_broadcast() should not be called from interrupt

     xc_unicast(flags, func, arg1, arg2, ci)
              Like xc_broadcast(), but call (*func)() on only the CPU
              indicated by ci.  xc_unicast() also returns a "ticket".

              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.

     The xcall interface is implemented within the file sys/kern/subr_xcall.c.

     kpreempt(9), percpu(9), softint(9)

     The xcall interface first appeared in NetBSD 5.0.

     Andrew Doran <ad@NetBSD.org>

NetBSD 8.99.34                 February 1, 2018                 NetBSD 8.99.34