Add a man page for DEFINE_IFUNC.
Reviewed by: kib Discussed with: emaste MFC after: 2 weeks Event: Waterloo Hackathon 2019 Differential Revision: https://reviews.freebsd.org/D20310
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Mark Johnston
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share/man/man9/DEFINE_IFUNC.9
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143
share/man/man9/DEFINE_IFUNC.9
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.\" Copyright (c) 2019 The FreeBSD Foundation
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.\"
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.\" This documentation was written by Mark Johnston <markj@FreeBSD.org>
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.\" under sponsorship from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd May 18, 2019
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.Dt DEFINE_IFUNC 9
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.Os
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.Sh NAME
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.Nm DEFINE_IFUNC
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.Nd define a kernel function with an implementation selected at run-time
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.Sh SYNOPSIS
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.In machine/ifunc.h
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.Fn DEFINE_IFUNC qual ret_type name args
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.Sh DESCRIPTION
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ifuncs are a linker feature which allows the programmer to define functions
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whose implementation is selected at boot-time or module load-time.
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The
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.Nm
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macro can be used to define an ifunc.
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The selection is performed by a resolver function, which returns a pointer
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to the selected function.
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ifunc resolvers are invoked very early during the machine-dependent
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initialization routine, or at load time for dynamically loaded modules.
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Resolution must occur before the first call to an ifunc.
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ifunc resolution is performed after CPU features are enumerated and after the
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kernel's environment is initialized.
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The typical use-case for an ifunc is a routine whose behavior depends on
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optional CPU features.
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For example, newer generations of a given CPU architecture may provide an
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instruction to optimize a common operation.
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To avoid the overhead of testing for the CPU feature each time the operation
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is performed, an ifunc can be used to provide two implementations for the
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operation: one targeting platforms with the extra instruction, and one
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for older platforms.
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.Pp
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Because
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.Nm
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is a macro that defines a dynamically typed function, its usage looks somewhat
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unusual.
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The
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.Ar qual
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parameter is a list of zero or more C function qualifiers to be applied to the
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ifunc.
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This parameter is typically empty or the
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.Dv static
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qualifier.
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.Ar ret_type
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is the return type of the ifunc.
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.Ar name
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is the name of the ifunc.
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.Ar args
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is a parenthesized, comma-separated list of the parameter types of the function,
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as they would appear in a C function declaration.
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.Pp
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The
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.Nm
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usage must be followed by the resolver function body.
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The resolver must return a function with return type
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.Ar ret_type
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and parameter types
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.Ar args .
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The resolver function is defined with the
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.Ql resolver
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gcc-style function attribute, causing the corresponding
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.Xr elf 5
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function symbol to be of type
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.Dv STT_GNU_IFUNC
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instead of
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.Dv STT_FUNC .
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The kernel linker invokes the resolver to process relocations targeting ifunc
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calls and PLT entries referencing such symbols.
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.Sh EXAMPLES
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ifunc resolvers are executed early during boot, before most kernel facilities
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are available.
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They are effectively limited to checking CPU feature flags and tunables.
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.Bd -literal
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static size_t
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fast_strlen(const char *s __unused)
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{
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size_t len;
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/* Fast, but may not be correct in all cases. */
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__asm("movq $42,%0\\n" : "=r" (len));
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return (len);
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}
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static size_t
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slow_strlen(const char *s)
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{
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const char *t;
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for (t = s; *t != '\\0'; t++);
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return (t - s);
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}
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DEFINE_IFUNC(, size_t, strlen, (const char *))
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{
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int enabled;
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enabled = 1;
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TUNABLE_INT_FETCH("debug.use_fast_strlen", &enabled);
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if (enabled && (cpu_features & CPUID_FAST_STRLEN) != 0)
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return (fast_strlen);
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else
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return (slow_strlen);
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}
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.Ed
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.Pp
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This defines a
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.Fn strlen
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function with an optimized implementation for CPUs that advertise support.
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.Sh SEE ALSO
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.Xr elf 5
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.Sh NOTES
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ifuncs are not supported on all architectures.
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They require both toolchain support, to emit function symbols of type
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.Dv STT_GNU_IFUNC ,
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and kernel linker support to invoke ifunc resolvers during boot or
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during module load.
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