[X][PATCH 1/2] x86/tsc: Add X86_FEATURE_TSC_KNOWN_FREQ flag

[Heitor R. Alves de Siqueira]

From: Bin Gao <bin.gao at linux.intel.com>

BugLink: https://bugs.launchpad.net/bugs/1822821

The X86_FEATURE_TSC_RELIABLE flag in Linux kernel implies both reliable
(at runtime) and trustable (at calibration). But reliable running and
trustable calibration independent of each other.

Add a new flag X86_FEATURE_TSC_KNOWN_FREQ, which denotes that the frequency
is known (via MSR/CPUID). This flag is only meant to skip the long term
calibration on systems which have a known frequency.

Add X86_FEATURE_TSC_KNOWN_FREQ to the skip the delayed calibration and
leave X86_FEATURE_TSC_RELIABLE in place.

After converting the existing users of X86_FEATURE_TSC_RELIABLE to use
either both flags or just X86_FEATURE_TSC_KNOWN_FREQ we can seperate the
functionality.

Suggested-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Bin Gao <bin.gao at intel.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Link: http://lkml.kernel.org/r/1479241644-234277-2-git-send-email-bin.gao@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
(cherry picked from commit 47c95a46d0fae07762f0a38aa3709ae63f307048)
Signed-off-by: Heitor R. Alves de Siqueira <halves at canonical.com>
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/kernel/tsc.c              | 11 ++++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index bd39531e12c9..1e13ff9bda0e 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -105,6 +105,7 @@
 #define X86_FEATURE_AMD_DCM     ( 3*32+27) /* multi-node processor */
 #define X86_FEATURE_APERFMPERF	( 3*32+28) /* APERFMPERF */
 #define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in S3 state */
+#define X86_FEATURE_TSC_KNOWN_FREQ ( 3*32+31) /* TSC has known frequency */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3	( 4*32+ 0) /* "pni" SSE-3 */
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 5927e11ad2c5..b66d4dadf9cf 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1222,10 +1222,15 @@ static int __init init_tsc_clocksource(void)
 		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
 
 	/*
-	 * Trust the results of the earlier calibration on systems
-	 * exporting a reliable TSC.
+	 * When TSC frequency is known (retrieved via MSR or CPUID), we skip
+	 * the refined calibration and directly register it as a clocksource.
+	 *
+	 * We still keep the TSC_RELIABLE flag here to avoid regressions -
+	 * it will be removed after all the conversion for other code paths
+	 * connected to this flag is done.
 	 */
-	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
+	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
+		boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
 		clocksource_register_khz(&clocksource_tsc, tsc_khz);
 		return 0;
 	}
-- 
2.21.0