From: Dexuan Cui <de...@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: 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. */
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
>
