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

[Dexuan Cui <decui@xxxxxxxxxxxxx>]

> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Sent: Thursday, June 20, 2024 2:19 PM
> To: Dexuan Cui <decui@xxxxxxxxxxxxx>; KY Srinivasan
> [...]
> > --- 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

Thanks for the good "comments"! :-)

I'm going to post v2 with the change below.

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e0fd57a8ba840..954b7cbfa2f02 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -449,9 +449,23 @@ 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.  */
+                               /*
+                                * Mark the Hyper-V TSC page feature as disabled
+                                * in a TDX VM without paravisor so that the
+                                * Invariant TSC, which is a better clocksource
+                                * anyway, is used instead.
+                                */
                                ms_hyperv.features &= ~HV_MSR_REFERENCE_TSC_AVAILABLE;

+                               /*
+                                * The Invariant TSC is expected to be available
+                                * in a TDX VM without paravisor, but if not,
+                                * print a warning message. The slower Hyper-V MSR-based
+                                * Ref Counter should end up being the clocksource.
+                                */
+                               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;