.\"	$NetBSD: pcu.9,v 1.2 2012/12/03 07:50:29 wiz Exp $
.\"
.\" Copyright (c) 2012 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 December 2, 2012
.Dt PCU 9
.Os
.Sh NAME
.Nm pcu
.Nd per-CPU unit (PCU)
.Sh SYNOPSIS
.In sys/pcu.h
.Ft void
.Fn pcu_load "const pcu_ops_t *pcu"
.Ft void
.Fn pcu_save "const pcu_ops_t *pcu"
.Ft void
.Fn pcu_save_all "lwp_t *l"
.Ft void
.Fn pcu_discard "const pcu_ops_t *pcu"
.Ft bool
.Fn pcu_used_p "const pcu_ops_t *pcu"
.\" -----
.Sh DESCRIPTION
Per CPU Unit (PCU) is an interface to manage synchronization of any
per-CPU context (unit) tied to an LWP context.
Typical use of PCU is for "lazy-switch" synchronisation of FPU state.
Each PCU has its operations defined by a
.Vt pcu_ops_t
structure.
Members of
.Vt pcu_ops_t
are
.Bd -literal
        u_int	pcu_id;
        void	(*pcu_state_save)(lwp_t *l);
        void	(*pcu_state_load)(lwp_t *l, bool used);
        void	(*pcu_state_release)(lwp_t *l);
.Ed
.Pp
.Bl -tag -width compact
.It Fn pcu_state_save
save the current CPU's state into the given LWP's MD storage.
.It Fn pcu_state_load
load PCU state from the given LWP's MD storage to the current CPU.
The
.Ar used
argument is true if it is not the first time the LWP uses the PCU.
.It Fn pcu_state_release
indicate to MD code that the PCU ownership by the LWP was released,
therefore the next use of PCU on the LWP shall be detected and
.Fn pcu_load
be called to reacquire ownership.
For example, this would often be the changing of a bit for a CPU to
trap on the execution of one of the PCU's instructions.
.El
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn pcu_load
Load (initialize) the PCU state of the current LWP on the current CPU.
.It Fn pcu_save
Save the PCU state to the given LWP.
.It Fn pcu_discard
Discard the PCU state of the current LWP.
.It Fn pcu_used_p
Return true if PCU was used (i.e.
.Fn pcu_load
was called) by the LWP.
Otherwise, return false.
.Fn pcu_save_all
save PCU state of the given LWP, so that it could be used later.
.El
.\" -----
.Sh CODE REFERENCES
.Nm
is implemented within the file
.Pa sys/kern/subr_pcu.c .
.Sh HISTORY
PCU first appeared in
.Nx 6.0 .