From: Dexuan Cui decui@microsoft.com Sent: Tuesday, June 18, 2024 5:25 PM

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: stable@vger.kernel.org # 6.6+ Signed-off-by: Dexuan Cui decui@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. */
  

I got confused by this comment, partly because I've forgotten the meaning of the ms_hyperv.feature flags. :-( Perhaps you could be more explicit in the comment and say "Mark the Hyper-V TSC page feature as disabled in a TDX VM so that the Invariant TSC, which is a better clocksource anyway, is used instead."

		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");
  

The above comment was even more confusing, because the code block doesn't do anything except print a message. The code doesn't force the use of the Ref Counter. I'd suggest something like: "The Invariant TSC is expected to be available, but if not, print a warning message. The slower Hyper-V MSR-based Ref Counter should end up being the clocksource."

Michael

• 	/* 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