Module Name: src
Committed By: maxv
Date: Sun Mar 11 13:38:02 UTC 2018
Modified Files:
src/sys/arch/x86/x86: cpu.c
Log Message:
Explain the TSC drift thing.
To generate a diff of this commit:
cvs rdiff -u -r1.149 -r1.150 src/sys/arch/x86/x86/cpu.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/x86/x86/cpu.c
diff -u src/sys/arch/x86/x86/cpu.c:1.149 src/sys/arch/x86/x86/cpu.c:1.150
--- src/sys/arch/x86/x86/cpu.c:1.149 Thu Feb 22 13:27:18 2018
+++ src/sys/arch/x86/x86/cpu.c Sun Mar 11 13:38:02 2018
@@ -1,4 +1,4 @@
-/* $NetBSD: cpu.c,v 1.149 2018/02/22 13:27:18 maxv Exp $ */
+/* $NetBSD: cpu.c,v 1.150 2018/03/11 13:38:02 maxv Exp $ */
/*
* Copyright (c) 2000-2012 NetBSD Foundation, Inc.
@@ -62,7 +62,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: cpu.c,v 1.149 2018/02/22 13:27:18 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: cpu.c,v 1.150 2018/03/11 13:38:02 maxv Exp $");
#include "opt_ddb.h"
#include "opt_mpbios.h" /* for MPDEBUG */
@@ -674,7 +674,7 @@ cpu_init(struct cpu_info *ci)
#endif /* MTRR */
if (ci != &cpu_info_primary) {
- /* Synchronize TSC again, and check for drift. */
+ /* Synchronize TSC */
wbinvd();
atomic_or_32(&ci->ci_flags, CPUF_RUNNING);
tsc_sync_ap(ci);
@@ -786,8 +786,9 @@ cpu_start_secondary(struct cpu_info *ci)
} else {
/*
* Synchronize time stamp counters. Invalidate cache and do
- * twice to try and minimize possible cache effects. Disable
- * interrupts to try and rule out any external interference.
+ * twice (in tsc_sync_bp) to minimize possible cache effects.
+ * Disable interrupts to try and rule out any external
+ * interference.
*/
psl = x86_read_psl();
x86_disable_intr();
@@ -854,19 +855,25 @@ cpu_hatch(void *v)
KDASSERT((ci->ci_flags & CPUF_PRESENT) == 0);
/*
- * Synchronize time stamp counters. Invalidate cache and do twice
- * to try and minimize possible cache effects. Note that interrupts
- * are off at this point.
+ * Synchronize the TSC for the first time. Note that interrupts are
+ * off at this point.
*/
wbinvd();
atomic_or_32(&ci->ci_flags, CPUF_PRESENT);
tsc_sync_ap(ci);
/*
- * Wait to be brought online. Use 'monitor/mwait' if available,
- * in order to make the TSC drift as much as possible. so that
- * we can detect it later. If not available, try 'pause'.
- * We'd like to use 'hlt', but we have interrupts off.
+ * Wait to be brought online.
+ *
+ * Use MONITOR/MWAIT if available. These instructions put the CPU in
+ * a low consumption mode (C-state), and if the TSC is not invariant,
+ * this causes the TSC to drift. We want this to happen, so that we
+ * can later detect (in tsc_tc_init) any abnormal drift with invariant
+ * TSCs. That's just for safety; by definition such drifts should
+ * never occur with invariant TSCs.
+ *
+ * If not available, try PAUSE. We'd like to use HLT, but we have
+ * interrupts off.
*/
while ((ci->ci_flags & CPUF_GO) == 0) {
if ((cpu_feature[1] & CPUID2_MONITOR) != 0) {
@@ -922,6 +929,11 @@ cpu_hatch(void *v)
lldt(GSYSSEL(GLDT_SEL, SEL_KPL));
ltr(ci->ci_tss_sel);
+ /*
+ * cpu_init will re-synchronize the TSC, and will detect any abnormal
+ * drift that would have been caused by the use of MONITOR/MWAIT
+ * above.
+ */
cpu_init(ci);
cpu_get_tsc_freq(ci);
