mirror of
https://github.com/jart/cosmopolitan.git
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66 lines
3.3 KiB
C
66 lines
3.3 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2022 Justine Alexandra Roberts Tunney │
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│ │
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│ Permission to use, copy, modify, and/or distribute this software for │
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│ any purpose with or without fee is hereby granted, provided that the │
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│ above copyright notice and this permission notice appear in all copies. │
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│ │
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
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│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
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│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
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│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
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│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
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│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
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│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
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│ PERFORMANCE OF THIS SOFTWARE. │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/dce.h"
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#include "libc/errno.h"
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#include "libc/intrin/fsgsbase.h"
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#include "libc/nexgen32e/x86feature.h"
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#ifdef __x86_64__
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/**
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* Returns true if FSGSBASE ISA can be used.
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*
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* If this function returns true (Linux 5.9+ or FreeBSD) then you should
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* be able to read/write to the %gs / %fs x86 segment registers in about
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* one or two clock cycles which gives you a free addition operation for
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* all assembly ops that reference memory.
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*
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* The FSGSBASE ISA was introduced by Intel with Ivybridge (c. 2012) but
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* the Linux Kernel didn't authorize us to use it until 2020, once Intel
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* had to start backdooring customer kernels so that they could have it.
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* AMD introduced support for the FSGSBASE ISA in Excavator, aka bdver4.
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*
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* @return boolean indicating if feature can be used
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* @see _rdfsbase()
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* @see _rdgsbase()
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* @see _wrfsbase()
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* @see _wrgsbase()
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*/
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privileged int _have_fsgsbase(void) {
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// Linux 5.9 (c. 2020) introduced close_range() and fsgsbase support.
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// it's cheaper to test for close_range() than handle an op crashing.
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// Windows lets us use these instructions but they don't really work.
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int ax;
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if (X86_HAVE(FSGSBASE)) {
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if (IsLinux()) {
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asm volatile("syscall"
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: "=a"(ax)
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: "0"(436 /* close_range */), "D"(-1), "S"(-2), "d"(0)
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: "rcx", "r11", "memory");
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return ax == -22; // EINVAL
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} else if (IsFreebsd()) {
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return 1;
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} else {
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return 0;
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}
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} else {
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return 0;
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}
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}
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#endif /* __x86_64__ */
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