clocksource: hyper-v: Use lapic timer in a TDX VM without paravisor

In a TDX VM without paravisor, currently the default timer is the Hyper-V
timer, which depends on the slow VM Reference Counter MSR: the Hyper-V TSC
page is not enabled in such a VM because the VM uses Invariant TSC as a
better clocksource and it's challenging to mark the Hyper-V TSC page shared
in very early boot.

Lower the rating of the Hyper-V timer so the local APIC timer becomes the
the default timer in such a VM, and print a warning in case Invariant TSC
is unavailable in such a VM. This change should cause no perceivable
performance difference.

Cc: [email protected] # 6.6+
Reviewed-by: Roman Kisel <[email protected]>
Signed-off-by: Dexuan Cui <[email protected]>
Reviewed-by: Michael Kelley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Wei Liu <[email protected]>
Message-ID: <[email protected]>

1 files changed, 15 insertions, 1 deletions

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index b2a080647e41..99177835cade 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -137,7 +137,21 @@ static int hv_stimer_init(unsigned int cpu)
 	ce->name = "Hyper-V clockevent";
 	ce->features = CLOCK_EVT_FEAT_ONESHOT;
 	ce->cpumask = cpumask_of(cpu);
-	ce->rating = 1000;
+
+	/*
+	 * Lower the rating of the Hyper-V timer in a TDX VM without paravisor,
+	 * so the local APIC timer (lapic_clockevent) is the default timer in
+	 * such a VM. The Hyper-V timer is not preferred in such a VM because
+	 * it depends on the slow VM Reference Counter MSR (the Hyper-V TSC
+	 * page is not enbled in such a VM because the VM uses Invariant TSC
+	 * as a better clocksource and it's challenging to mark the Hyper-V
+	 * TSC page shared in very early boot).
+	 */
+	if (!ms_hyperv.paravisor_present && hv_isolation_type_tdx())
+		ce->rating = 90;
+	else
+		ce->rating = 1000;
+
 	ce->set_state_shutdown = hv_ce_shutdown;
 	ce->set_state_oneshot = hv_ce_set_oneshot;
 	ce->set_next_event = hv_ce_set_next_event;