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.\"	$NetBSD: pcu.9,v 1.12 2017/03/17 10:09:24 wiz Exp $
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.\" by Mindaugas Rasiukevicius.
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.Dd May 16, 2017
.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" "lwp_t *l"
.Ft void
.Fn pcu_save_all "lwp_t *l"
.Ft void
.Fn pcu_discard "const pcu_ops_t *pcu" "lwp_t *l" "bool valid"
.Ft void
.Fn pcu_discard_all "lwp_t *l"
.Ft bool
.Fn pcu_valid_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" synchronization of the 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, u_int flags);
        void	(*pcu_state_release)(lwp_t *l);
.Ed
.Pp
.Bl -tag
.It Fn pcu_state_save
Indicate to MD code that the PCU state on the current CPU should be
saved 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 flags
argument is a combination of one or more of the following:
.Bl -tag -width PCU_VALIDXXX
.It Dv PCU_VALID
Indicate that the PCU state is considered valid and need not be initialized.
This is the case if the PCU state was already used (and thus loaded) by the LWP
and has not been discarded since.
.It Dv PCU_REENABLE
Indicate that a fault reoccurred while the PCU state is loaded,
therefore PCU should be re-enabled.
This happens if LWP is context switched to another CPU and then switched
back to the original CPU while the state on that CPU has not been changed
by other LWPs.
It may also happen due to instruction "bouncing" on some architectures.
.El
.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 the ownership.
For example, this would normally 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 pcu_save_allXXX
.It Fn pcu_load
Load or initialize the PCU state of the current LWP on the current CPU.
.It Fn pcu_save
Save the specified PCU state to the given LWP.
.It Fn pcu_discard
Discard the specified PCU state of the current LWP.
The PCU state will be considered invalid,
unless the "valid" parameter is set to true.
.It Fn pcu_valid_p
Return true if PCU state is considered valid.
Generally, it always becomes "valid" when
.Fn pcu_load
is called by the LWP.
Otherwise, return false.
.It Fn pcu_discard_all
Discard all PCU states of the given LWP; generally used by exec and exit.
.It Fn pcu_save_all
Save all PCU states of the given LWP; generally used during new LWP
creation so that the PCU state of the parent could be copied.
.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 .
.Sh AUTHORS
.An Mindaugas Rasiukevicius Aq Mt rmind@NetBSD.org