__ppc_yield.3: Document PPC performance-hint functions

GNU C Library 2.18 adds functions __ppc_yield(3), __ppc_mdoio(3),
and __ppc_mdoom(3) that can be used provide a hint that
performance could be improved if shared resources are released
for use by other processors.

Signed-off-by: Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

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+.\" Copyright (c) 2015, IBM Corporation.
+.\"
+.\" %%%LICENSE_START(VERBATIM)
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+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
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+.\" Formatted or processed versions of this manual, if unaccompanied by
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+.\" Process this file with
+.\" groff -man -Tascii foo.1
+.\"
+.TH __PPC_YIELD 3 2015-08-05 "GNU C Library" "Linux Programmer's\
+Manual"
+.SH NAME
+__ppc_yield, __ppc_mdoio, __ppc_mdoom \-
+Hint the processor to release shared resources
+.SH SYNOPSIS
+.B #include <sys/platform/ppc.h>
+.sp
+.B void __ppc_yield(void);
+.br
+.B void __ppc_mdoio(void);
+.br
+.B void __ppc_mdoom(void);
+.SH DESCRIPTION
+The functions
+.BR __ppc_yield (),
+.BR __ppc_mdoio ()
+and
+.BR __ppc_mdoom ()
+provide hints about the usage of resources that are shared with other
+processors.
+They can be used, for example, if a program waiting on a lock intends
+to divert the shared resources to be used by other processors.
+More information is available in Power ISA 2.06b - Book II - Section
+3.2.
+.P
+.BR __ppc_yield ()
+provides a hint that performance will probably be improved if shared
+resources dedicated to the executing processor are released for use by
+other processors.
+.P
+.BR __ppc_mdoio ()
+provides a hint that performance will probably be improved if shared
+resources dedicated to the executing processor are released until all
+outstanding storage accesses to caching-inhibited storage have been
+completed.
+.P
+.BR __ppc_mdoom ()
+provides a hint that performance will probably be improved if shared
+resources dedicated to the executing processor are released until all
+outstanding storage accesses to cacheable storage for which the data
+is not in the cache have been completed.
+.SH VERSIONS
+GNU C Library added support for
+.BR __ppc_yield (),
+.BR __ppc_mdoio ()
+and
+.BR __ppc_mdoom ()
+in version 2.18.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.ad l
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface      Attribute       Value
+T{
+.BR __ppc_yield (),
+.BR __ppc_mdoio (),
+.BR __ppc_mdoom ()
+T}     Thread safety   MT-Safe
+.TE
+.ad
+.SH CONFORMING TO
+These functions are nonstandard GNU extensions.
+.SH SEE ALSO
+.BR __ppc_set_ppr_med (3)