Date: Mon, 20 May 2019 19:12:29 +0000 (UTC) From: Mark Johnston <markj@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r348003 - head/share/man/man9 Message-ID: <201905201912.x4KJCTUT012079@repo.freebsd.org>
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Author: markj Date: Mon May 20 19:12:29 2019 New Revision: 348003 URL: https://svnweb.freebsd.org/changeset/base/348003 Log: 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 Added: head/share/man/man9/DEFINE_IFUNC.9 (contents, props changed) Added: head/share/man/man9/DEFINE_IFUNC.9 ============================================================================== --- /dev/null 00:00:00 1970 (empty, because file is newly added) +++ head/share/man/man9/DEFINE_IFUNC.9 Mon May 20 19:12:29 2019 (r348003) @@ -0,0 +1,143 @@ +.\" Copyright (c) 2019 The FreeBSD Foundation +.\" +.\" This documentation was written by Mark Johnston <markj@FreeBSD.org> +.\" under sponsorship from the FreeBSD Foundation. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" $FreeBSD$ +.\" +.Dd May 18, 2019 +.Dt DEFINE_IFUNC 9 +.Os +.Sh NAME +.Nm DEFINE_IFUNC +.Nd define a kernel function with an implementation selected at run-time +.Sh SYNOPSIS +.In machine/ifunc.h +.Fn DEFINE_IFUNC qual ret_type name args +.Sh DESCRIPTION +ifuncs are a linker feature which allows the programmer to define functions +whose implementation is selected at boot-time or module load-time. +The +.Nm +macro can be used to define an ifunc. +The selection is performed by a resolver function, which returns a pointer +to the selected function. +ifunc resolvers are invoked very early during the machine-dependent +initialization routine, or at load time for dynamically loaded modules. +Resolution must occur before the first call to an ifunc. +ifunc resolution is performed after CPU features are enumerated and after the +kernel's environment is initialized. +The typical use-case for an ifunc is a routine whose behavior depends on +optional CPU features. +For example, newer generations of a given CPU architecture may provide an +instruction to optimize a common operation. +To avoid the overhead of testing for the CPU feature each time the operation +is performed, an ifunc can be used to provide two implementations for the +operation: one targeting platforms with the extra instruction, and one +for older platforms. +.Pp +Because +.Nm +is a macro that defines a dynamically typed function, its usage looks somewhat +unusual. +The +.Ar qual +parameter is a list of zero or more C function qualifiers to be applied to the +ifunc. +This parameter is typically empty or the +.Dv static +qualifier. +.Ar ret_type +is the return type of the ifunc. +.Ar name +is the name of the ifunc. +.Ar args +is a parenthesized, comma-separated list of the parameter types of the function, +as they would appear in a C function declaration. +.Pp +The +.Nm +usage must be followed by the resolver function body. +The resolver must return a function with return type +.Ar ret_type +and parameter types +.Ar args . +The resolver function is defined with the +.Ql resolver +gcc-style function attribute, causing the corresponding +.Xr elf 5 +function symbol to be of type +.Dv STT_GNU_IFUNC +instead of +.Dv STT_FUNC . +The kernel linker invokes the resolver to process relocations targeting ifunc +calls and PLT entries referencing such symbols. +.Sh EXAMPLES +ifunc resolvers are executed early during boot, before most kernel facilities +are available. +They are effectively limited to checking CPU feature flags and tunables. +.Bd -literal +static size_t +fast_strlen(const char *s __unused) +{ + size_t len; + + /* Fast, but may not be correct in all cases. */ + __asm("movq $42,%0\\n" : "=r" (len)); + return (len); +} + +static size_t +slow_strlen(const char *s) +{ + const char *t; + + for (t = s; *t != '\\0'; t++); + return (t - s); +} + +DEFINE_IFUNC(, size_t, strlen, (const char *)) +{ + int enabled; + + enabled = 1; + TUNABLE_INT_FETCH("debug.use_fast_strlen", &enabled); + if (enabled && (cpu_features & CPUID_FAST_STRLEN) != 0) + return (fast_strlen); + else + return (slow_strlen); +} +.Ed +.Pp +This defines a +.Fn strlen +function with an optimized implementation for CPUs that advertise support. +.Sh SEE ALSO +.Xr elf 5 +.Sh NOTES +ifuncs are not supported on all architectures. +They require both toolchain support, to emit function symbols of type +.Dv STT_GNU_IFUNC , +and kernel linker support to invoke ifunc resolvers during boot or +during module load.
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