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

NAME
     pcu -- per-CPU unit (PCU)

SYNOPSIS
     #include <sys/pcu.h>

     void
     pcu_load(const pcu_ops_t *pcu);

     void
     pcu_save(const pcu_ops_t *pcu);

     void
     pcu_save_all(lwp_t *l);

     void
     pcu_discard(const pcu_ops_t *pcu, bool valid);

     void
     pcu_discard_all(lwp_t *l);

     bool
     pcu_valid_p(const pcu_ops_t *pcu);

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 pcu_ops_t structure.  Members of pcu_ops_t are

             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);

     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.

     pcu_state_load()  Load PCU state from the given LWP's MD storage to the
                       current CPU.  The flags argument is a combination of
                       one or more of the following:

                       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.

                       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.

     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 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.

FUNCTIONS
     pcu_load()       Load or initialize the PCU state of the current LWP on
                      the current CPU.

     pcu_save()       Save the specified PCU state to the given LWP.

     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.

     pcu_valid_p()    Return true if PCU state is considered valid.
                      Generally, it always becomes "valid" when pcu_load() is
                      called by the LWP.  Otherwise, return false.

     pcu_discard_all()
                      Discard all PCU states of the given LWP; generally used
                      by exec and exit.

     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.

CODE REFERENCES
     pcu is implemented within the file sys/kern/subr_pcu.c.

HISTORY
     PCU first appeared in NetBSD 6.0.

AUTHORS
     Mindaugas Rasiukevicius <rmind@NetBSD.org>

NetBSD 7.1.2                     May 25, 2014                     NetBSD 7.1.2