Re: [PATCH v15 10/13] tsc: Upgrade TSC clocksource rating

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 1/2/2025 3:02 PM, Borislav Petkov wrote:
> On Thu, Jan 02, 2025 at 10:50:53AM +0530, Nikunj A. Dadhania wrote:
>> This is what was suggested by tglx: 
>>
>> "So if you know you want TSC to be selected, then upgrade the rating of
>>  both the early and the regular TSC clocksource and be done with it."
> 
> I highly doubt that he saw what you have now:
> 
> Your commit message is talking about virtualized environments but your diff is
> doing a global, unconditional change which affects *everything*.

Right, let me limit this only to virtualized environments as part of 
CONFIG_PARAVIRT.

Subject: [PATCH] x86/tsc: Upgrade TSC clocksource rating for guests

Hypervisor platform setup (x86_hyper_init::init_platform) routines register 
their own PV clock sources (KVM, HyperV, and Xen) at different clock ratings
resulting in selection of PV clock source even though a stable TSC clock
source is available. Upgrade the clock rating of the TSC early and
regular clock source to prefer TSC over PV clock sources when TSC is
invariant, non-stop and stable

Cc: Alexey Makhalov <alexey.makhalov@broadcom.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
 arch/x86/kernel/tsc.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 34dec0b72ea8..5c6831a42889 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -274,10 +274,31 @@ bool using_native_sched_clock(void)
 {
 	return static_call_query(pv_sched_clock) == native_sched_clock;
 }
+
+/*
+ * Upgrade the clock rating for TSC early and regular clocksource when the
+ * underlying platform provides non-stop, invariant, and stable TSC. TSC
+ * early/regular clocksource will be preferred over other para-virtualized clock
+ * sources.
+ */
+static void __init upgrade_clock_rating(struct clocksource *tsc_early,
+					struct clocksource *tsc)
+{
+	if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR) &&
+	    cpu_feature_enabled(X86_FEATURE_CONSTANT_TSC) &&
+	    cpu_feature_enabled(X86_FEATURE_NONSTOP_TSC) &&
+	    !tsc_unstable) {
+		tsc_early->rating = 449;
+		tsc->rating = 450;
+	}
+}
 #else
 u64 sched_clock_noinstr(void) __attribute__((alias("native_sched_clock")));
 
 bool using_native_sched_clock(void) { return true; }
+
+static void __init upgrade_clock_rating(struct clocksource *tsc_early,
+					struct clocksource *tsc) { }
 #endif
 
 notrace u64 sched_clock(void)
@@ -1564,6 +1585,8 @@ void __init tsc_init(void)
 	if (tsc_clocksource_reliable || no_tsc_watchdog)
 		tsc_disable_clocksource_watchdog();
 
+	upgrade_clock_rating(&clocksource_tsc_early, &clocksource_tsc);
+
 	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
 	detect_art();
 }
-- 
2.34.1