linux-kernel - RE: [PATCH] clocksource: hyper-v: Prefer architecture counter when running as root partition

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Message-ID:
 <SN6PR02MB41578685EF8F664D77DF2156D42CA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Thu, 7 Aug 2025 20:27:32 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "wei.liu@...nel.org" <wei.liu@...nel.org>, Linux on Hyper-V List
	<linux-hyperv@...r.kernel.org>
CC: "K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang
	<haiyangz@...rosoft.com>, Dexuan Cui <decui@...rosoft.com>, 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:X86
 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clocksource: hyper-v: Prefer architecture counter when
 running as root partition

From: wei.liu@...nel.org <wei.liu@...nel.org> Sent: Thursday, August 7, 2025 9:59 AM
> 
> There is no HV_ACCESS_TSC_INVARIANT bit when Linux runs as the root
> partition. 

Some clarifying questions here: When you say "there is no
HV_ACCESS_TSC_INVARIANT bit", does that mean that bit 15 of the
HV_PARTITION_PRIVILEGE_MASK is just unused and undefined?

And what is the behavior if the root partition writes to
HV_X64_MSR_TSC_INVARIANT_CONTROL? In a normal x86 guest,
HV_X64_MSR_TSC_INVARIANT_CONTROL determines whether
CPUID 0x80000007/EDX bit 8 is set. What will the root partition see
for CPUID 0x80000007/EDX bit 8? Whatever the underlying hardware
provides? See also the comment in ms_hyperv_init_platform().

Michael

> The old logic caused the native TSC clock source to be
> incorrectly marked as unstable on x86.
> 
> The clock source driver runs on both x86 and ARM64. Change it to prefer
> architectural counter when it runs on Linux root.
> 
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> ---
> Cc: Michael Kelley <mhklinux@...look.com>
> 
> Pending further testing.
> 
> The preference of architectural counter over Hyper-V Reference TSC for
> Linux root is confirmed by the hypervisor team.
> ---
>  arch/x86/kernel/cpu/mshyperv.c     |  6 +++++-
>  drivers/clocksource/hyperv_timer.c | 10 +++++++++-
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index fd708180d2d9..1713545dcf4a 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -966,8 +966,12 @@ static void __init ms_hyperv_init_platform(void)
>  	 * TSC should be marked as unstable only after Hyper-V
>  	 * clocksource has been initialized. This ensures that the
>  	 * stability of the sched_clock is not altered.
> +	 *
> +	 * The root partition doesn't see HV_ACCESS_TSC_INVARIANT.
> +	 * No need to check for it.
>  	 */
> -	if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
> +	if (!hv_root_partition() &&
> +	    !(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
>  		mark_tsc_unstable("running on Hyper-V");
> 
>  	hardlockup_detector_disable();
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index f6415e726e96..59c3e09f1961 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -534,14 +534,22 @@ static void __init hv_init_tsc_clocksource(void)
>  	union hv_reference_tsc_msr tsc_msr;
> 
>  	/*
> +	 * When running as a guest partition:
> +	 *
>  	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
>  	 * handles frequency and offset changes due to live migration,
>  	 * pause/resume, and other VM management operations.  So lower the
>  	 * Hyper-V Reference TSC rating, causing the generic TSC to be used.
>  	 * TSC_INVARIANT is not offered on ARM64, so the Hyper-V Reference
>  	 * TSC will be preferred over the virtualized ARM64 arch counter.
> +	 *
> +	 * When running as the root partition:
> +	 *
> +	 * There is no HV_ACCESS_TSC_INVARIANT feature. Always prefer the
> +	 * architectural defined counter over the Hyper-V Reference TSC.
>  	 */
> -	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> +	if ((ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) ||
> +	    hv_root_partition()) {
>  		hyperv_cs_tsc.rating = 250;
>  		hyperv_cs_msr.rating = 245;
>  	}
> --
> 2.43.0