linux-kernel - [tip:x86/urgent] x86/tsc: Make the TSC ADJUST sanitizing work for tsc

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Date:   Fri, 10 Feb 2017 00:49:16 -0800
From:   tip-bot for Thomas Gleixner <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org, olof@...om.net,
        hpa@...or.com, tglx@...utronix.de, mingo@...nel.org
Subject: [tip:x86/urgent] x86/tsc: Make the TSC ADJUST sanitizing work for
 tsc_reliable

Commit-ID:  5f2e71e71410ecb858cfec184ba092adaca61626
Gitweb:     http://git.kernel.org/tip/5f2e71e71410ecb858cfec184ba092adaca61626
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Thu, 9 Feb 2017 16:08:42 +0100
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 10 Feb 2017 09:47:17 +0100

x86/tsc: Make the TSC ADJUST sanitizing work for tsc_reliable

When the TSC is marked reliable then the synchronization check is skipped,
but that also skips the TSC ADJUST sanitizing code. So on a machine with a
wreckaged BIOS the TSC deviation between CPUs might go unnoticed.

Let the TSC adjust sanitizing code run unconditionally and just skip the
expensive synchronization checks when TSC is marked reliable.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Olof Johansson <olof@...om.net>
Link: http://lkml.kernel.org/r/20170209151231.491189912@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 arch/x86/kernel/tsc_sync.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index d0db011..728f753 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -286,13 +286,6 @@ void check_tsc_sync_source(int cpu)
 	if (unsynchronized_tsc())
 		return;
 
-	if (tsc_clocksource_reliable) {
-		if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
-			pr_info(
-			"Skipped synchronization checks as TSC is reliable.\n");
-		return;
-	}
-
 	/*
 	 * Set the maximum number of test runs to
 	 *  1 if the CPU does not provide the TSC_ADJUST MSR
@@ -380,14 +373,19 @@ void check_tsc_sync_target(void)
 	int cpus = 2;
 
 	/* Also aborts if there is no TSC. */
-	if (unsynchronized_tsc() || tsc_clocksource_reliable)
+	if (unsynchronized_tsc())
 		return;
 
 	/*
 	 * Store, verify and sanitize the TSC adjust register. If
 	 * successful skip the test.
+	 *
+	 * The test is also skipped when the TSC is marked reliable. This
+	 * is true for SoCs which have no fallback clocksource. On these
+	 * SoCs the TSC is frequency synchronized, but still the TSC ADJUST
+	 * register might have been wreckaged by the BIOS..
 	 */
-	if (tsc_store_and_check_tsc_adjust(false)) {
+	if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) {
 		atomic_inc(&skip_test);
 		return;
 	}