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

[Dexuan Cui]

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. This change should cause no perceivable
performance difference.

Cc: sta...@vger.kernel.org # 6.6+
Signed-off-by: Dexuan Cui <de...@microsoft.com>
---
 arch/x86/kernel/cpu/mshyperv.c     |  6 +++++-
 drivers/clocksource/hyperv_timer.c | 16 +++++++++++++++-
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e0fd57a8ba840..745af47ca0459 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -449,9 +449,13 @@ static void __init ms_hyperv_init_platform(void)
                        ms_hyperv.hints &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
 
                        if (!ms_hyperv.paravisor_present) {
-                               /* To be supported: more work is required.  */
+                               /* Use Invariant TSC as a better clocksource. */
                                ms_hyperv.features &= 
~HV_MSR_REFERENCE_TSC_AVAILABLE;
 
+                               /* Use the Ref Counter in case Invariant TSC is 
unavailable. */
+                               if (!(ms_hyperv.features & 
HV_ACCESS_TSC_INVARIANT))
+                                       pr_warn("Hyper-V: Invariant TSC is 
unavailable\n");
+
                                /* HV_MSR_CRASH_CTL is unsupported. */
                                ms_hyperv.misc_features &= 
~HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
 
diff --git a/drivers/clocksource/hyperv_timer.c 
b/drivers/clocksource/hyperv_timer.c
index b2a080647e413..99177835cadec 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;
-- 
2.25.1