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Date:      Thu, 7 Feb 2019 02:56:10 +0000 (UTC)
From:      Konstantin Belousov <kib@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r343855 - head/lib/libc/x86/sys
Message-ID:  <201902070256.x172uAQH093939@repo.freebsd.org>
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Author: kib
Date: Thu Feb  7 02:56:10 2019
New Revision: 343855
URL: https://svnweb.freebsd.org/changeset/base/343855

Log:
  Use ifunc to select the barrier instruction for RDTSC.
  
  This optimizes out runtime switch and removes yet another cpuid from
  libc.
  
  Note that this is the first use of ifunc in i386 libc, so
  ifunc-capable toolchain is required for building runnable userspace on
  i386, same as on amd64.
  
  Discussed with:	emaste
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

Modified:
  head/lib/libc/x86/sys/__vdso_gettc.c

Modified: head/lib/libc/x86/sys/__vdso_gettc.c
==============================================================================
--- head/lib/libc/x86/sys/__vdso_gettc.c	Thu Feb  7 02:36:59 2019	(r343854)
+++ head/lib/libc/x86/sys/__vdso_gettc.c	Thu Feb  7 02:56:10 2019	(r343855)
@@ -1,6 +1,6 @@
 /*-
  * Copyright (c) 2012 Konstantin Belousov <kib@FreeBSD.org>
- * Copyright (c) 2016, 2017 The FreeBSD Foundation
+ * Copyright (c) 2016, 2017, 2019 The FreeBSD Foundation
  * All rights reserved.
  *
  * Portions of this software were developed by Konstantin Belousov
@@ -50,15 +50,9 @@ __FBSDID("$FreeBSD$");
 #ifdef WANT_HYPERV
 #include <dev/hyperv/hyperv.h>
 #endif
+#include <x86/ifunc.h>
 #include "libc_private.h"
 
-static enum LMB {
-	LMB_UNKNOWN,
-	LMB_NONE,
-	LMB_MFENCE,
-	LMB_LFENCE
-} lfence_works = LMB_UNKNOWN;
-
 static void
 cpuidp(u_int leaf, u_int p[4])
 {
@@ -84,68 +78,35 @@ cpuidp(u_int leaf, u_int p[4])
 	    :  "0" (leaf));
 }
 
-static enum LMB
-select_lmb(void)
+static void
+rdtsc_mb_lfence(void)
 {
-	u_int p[4];
-	static const char intel_id[] = "GenuntelineI";
 
-	cpuidp(0, p);
-	return (memcmp(p + 1, intel_id, sizeof(intel_id) - 1) == 0 ?
-	    LMB_LFENCE : LMB_MFENCE);
+	lfence();
 }
 
 static void
-init_fence(void)
+rdtsc_mb_mfence(void)
 {
-#if defined(__i386__)
-	u_int cpuid_supported, p[4];
 
-	lfence_works = LMB_NONE;
-	__asm __volatile(
-	    "	pushfl\n"
-	    "	popl	%%eax\n"
-	    "	movl    %%eax,%%ecx\n"
-	    "	xorl    $0x200000,%%eax\n"
-	    "	pushl	%%eax\n"
-	    "	popfl\n"
-	    "	pushfl\n"
-	    "	popl    %%eax\n"
-	    "	xorl    %%eax,%%ecx\n"
-	    "	je	1f\n"
-	    "	movl	$1,%0\n"
-	    "	jmp	2f\n"
-	    "1:	movl	$0,%0\n"
-	    "2:\n"
-	    : "=r" (cpuid_supported) : : "eax", "ecx", "cc");
-	if (cpuid_supported) {
-		cpuidp(0x1, p);
-		if ((p[3] & CPUID_SSE2) != 0)
-			lfence_works = select_lmb();
-	}
-#elif defined(__amd64__)
-	lfence_works = select_lmb();
-#else
-#error "Arch"
-#endif
+	mfence();
 }
 
 static void
-rdtsc_mb(void)
+rdtsc_mb_none(void)
 {
+}
 
-again:
-	if (__predict_true(lfence_works == LMB_LFENCE)) {
-		lfence();
-		return;
-	} else if (lfence_works == LMB_MFENCE) {
-		mfence();
-		return;
-	} else if (lfence_works == LMB_NONE) {
-		return;
-	}
-	init_fence();
-	goto again;
+DEFINE_UIFUNC(static, void, rdtsc_mb, (void), static)
+{
+	u_int p[4];
+	static const char intel_id[] = "GenuntelineI";
+
+	if ((cpu_feature & CPUID_SSE2) == 0)
+		return (rdtsc_mb_none);
+	cpuidp(0, p);
+	return (memcmp(p + 1, intel_id, sizeof(intel_id) - 1) == 0 ?
+	    rdtsc_mb_lfence : rdtsc_mb_mfence);
 }
 
 static u_int

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