.\" $NetBSD: ipi.9,v 1.4.2.1 2020/09/13 12:20:22 martin Exp $
.\"
.\" Copyright (c) 2014, 2019 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Mindaugas Rasiukevicius.
.\"
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.Dd March 31, 2019
.Dt IPI 9
.Os
.Sh NAME
.Nm ipi
.Nd machine-independent interprocessor interrupts
.Sh SYNOPSIS
.In sys/ipi.h
.Vt typedef void (*ipi_func_t)(void *);
.\" -----
.Ft u_int
.Fn ipi_register "ipi_func_t func" "void *arg"
.Ft void
.Fn ipi_unregister "u_int ipi_id"
.Ft void
.Fn ipi_trigger "u_int ipi_id" "struct cpu_info *ci"
.Ft void
.Fn ipi_trigger_multi "u_int ipi_id" "const kcpuset_t *target"
.Ft void
.Fn ipi_trigger_broadcast "u_int ipi_id" "bool skip_self"
.\" -----
.Ft void
.Fn ipi_unicast "ipi_msg_t *msg" "struct cpu_info *ci"
.Ft void
.Fn ipi_multicast "ipi_msg_t *msg" "const kcpuset_t *target"
.Ft void
.Fn ipi_broadcast "ipi_msg_t *msg" "bool skip_self"
.Ft void
.Fn ipi_wait "ipi_msg_t *msg"
.\" -----
.Sh DESCRIPTION
The machine-independent
.Nm
interface provides capability to send inter-processor interrupts (IPIs)
amongst CPUs.
The interface has two mechanisms: asynchronous IPI to invoke functions
with a constant argument and synchronous IPIs with the cross-call support.
.Pp
Other synchronization interfaces are built using the MI IPI interface.
For a general purpose inter-processor cross-calls or remote
interrupts, use the
.Xr xcall 9
or
.Xr softint 9
interfaces.
.Pp
The primary use cases of the MI IPIs include the following:
.Bl -hyphen -compact
.It
provide a facility for the
.Xr softint 9
subsystem to schedule software interrupts on remote CPUs
.It
provide a facility for the
.Xr xcall 9
subsystem
.It
abstract IPI handling and facilitate machine-dependent code
.El
.\" -----
.Ss Asynchronous IPI interface
This interface allows dynamic registration of IPI handlers with a constant
argument and asynchronous triggering of interrupts.
.Bl -tag -width compact
.It Fn ipi_register "func" "arg"
Register an IPI handler
.Fa func
with an arbitrary argument
.Fa arg .
Returns a non-zero IPI identifier on success and zero on failure.
.It Fn ipi_unregister "ipi_id"
Unregister the IPI handler identified by the
.Fa ipi_id .
.It Fn ipi_trigger "ipi_id" "ci"
Trigger an IPI identified by
.Fa ipi_id
on a remote CPU specified by
.Fa ci .
This function must be called with kernel preemption disabled and
the target CPU must be remote.
.It Fn ipi_trigger_multi "ipi_id" "target"
Trigger an IPI identified by
.Fa ipi_id
on all of the CPUs in the set specified by
.Fa target .
This function must be called with kernel preemption disabled.
The sending CPU may be included in the CPU set; when this is the case,
the IPI on the sending CPU is processes synchronously.
.It Fn ipi_trigger_broadcast "ipi_id" "skip_self"
Trigger an IPI identified by
.Fa ipi_id
on all of the attached CPUs.
This function must be called with kernel preemption disabled.
Optionally, the sending CPU may be skipped by passing
.Dv true
for
.Fa skip_self .
.El
.\" -----
.Ss Synchronous IPI interface
This interface provides capability to perform cross-calls, i.e. invoke
an arbitrary function on a remote CPU.
The invocations are performed synchronously and the caller must wait
for completion.
The cross-call is described by an IPI "message".
The caller has to fill in an
.Vt ipi_msg_t
structure which has the following public members:
.Bd -literal
        ipi_func_t	func;
        void		arg;
.Ed
.Pp
The
.Ar func
member specifies a function to invoke and
.Ar arg
is the argument to be passed to the function.
.Bl -tag -width compact
.It Fn ipi_unicast "msg" "ci"
Send an IPI to a remote CPU specified by
.Fa ci .
.It Fn ipi_multicast "msg" "target"
Send IPIs to a CPU set specified by
.Fa target .
.It Fn ipi_broadcast "msg" "skip_self"
Send IPIs to all CPUs.
Optionally, the sending CPU may be skipped by passing
.Dv true
for
.Fa skip_self .
.It Fn ipi_wait "msg"
Wait until all IPIs complete.
.El
.Pp
All described functions, except
.Fn ipi_wait ,
must be called with the kernel preemption disabled.
All synchronous IPI invocations must be completed (wait for them with the
.Fn ipi_wait
function) before the IPI message structure can be destroyed or new
cross-call requests can be performed.
.\" -----
.Sh NOTES
Functions being called must be lightweight.
They run at
.Dv IPL_HIGH
and should generally not use any other synchronization interfaces
such as
.Xr mutex 9 .
If spin-locks are used, they must be used carefully and have no contention.
.\" -----
.Sh CODE REFERENCES
The
.Nm
interface is implemented within the file
.Pa sys/kern/subr_ipi.c .
.\" -----
.Sh SEE ALSO
.Xr kcpuset 9 ,
.Xr kpreempt 9 ,
.Xr softint 9 ,
.Xr spl 9 ,
.Xr xcall 9
.Sh HISTORY
The
.Nm
interface first appeared in
.Nx 7.0 .
.Sh AUTHORS
.An Mindaugas Rasiukevicius Aq Mt rmind@NetBSD.org