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

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Date: Thu, 20 Jun 2024 21:18:52 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Dexuan Cui <decui@...rosoft.com>, "K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Thomas
 Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter
 Anvin" <hpa@...or.com>, Daniel Lezcano <daniel.lezcano@...aro.org>, "open
 list:Hyper-V/Azure CORE AND DRIVERS" <linux-hyperv@...r.kernel.org>, "open
 list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>
CC: "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] clocksource: hyper-v: Use lapic timer in a TDX VM without
 paravisor

From: Dexuan Cui <decui@...rosoft.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@...r.kernel.org # 6.6+
> Signed-off-by: Dexuan Cui <decui@...rosoft.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
>