linux-kernel - RE: [PATCH v2] clocksource: hyper-v: Use lapic timer in a TDX VM without paravisor

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Date: Fri, 21 Jun 2024 15:18:31 +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>, Roman Kisel
	<romank@...ux.microsoft.com>
Subject: RE: [PATCH v2] clocksource: hyper-v: Use lapic timer in a TDX VM
 without paravisor

From: Dexuan Cui <decui@...rosoft.com> Sent: Thursday, June 20, 2024 11:16 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, and print a warning in case Invariant TSC
> is unavailable in such a VM. This change should cause no perceivable
> performance difference.
> 
> Cc: stable@...r.kernel.org # 6.6+
> Reviewed-by: Roman Kisel <romank@...ux.microsoft.com>
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> ---
> 
> Changes in v2:
>     Improved the comments in ms_hyperv_init_platform() [Michael Kelley]
>     Added "print a warning in case Invariant TSC  unavailable" in the changelog.
>     Added Roman's Reviewed-by.
> 
>  arch/x86/kernel/cpu/mshyperv.c     | 16 +++++++++++++++-
>  drivers/clocksource/hyperv_timer.c | 16 +++++++++++++++-
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> 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;
> 
> 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

Reviewed-by: Michael Kelley <mhklinux@...look.com>