Re: [PATCH v2] clocksource: hyper-v: Skip unnecessary checks for the root partition

[Wei Liu <wei.liu@kernel.org>]

On Tue, Aug 12, 2025 at 07:48:45PM +0000, Wei Liu wrote:
> The HV_ACCESS_TSC_INVARIANT bit is always zero when Linux runs as the
> root partition. The root partition will see directly what the hardware
> provides.
> 
> The old logic in ms_hyperv_init_platform caused the native TSC clock
> source to be incorrectly marked as unstable on x86. Fix it.
> 
> Skip the unnecessary checks in code for the root partition. Add one
> extra comment in code to clarify the behavior.
> 
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> ---
> v2: update the commit message and comments
> ---
>  arch/x86/kernel/cpu/mshyperv.c     | 11 ++++++++++-
>  drivers/clocksource/hyperv_timer.c | 10 +++++++++-
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index c78f860419d6..25773af116bc 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -565,6 +565,11 @@ static void __init ms_hyperv_init_platform(void)
>  	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
>  #endif
>  #endif
> +	/*
> +	 * HV_ACCESS_TSC_INVARIANT is always zero for the root partition. Root
> +	 * partition doesn't need to write to synthetic MSR to enable invariant
> +	 * TSC feature. It sees what the hardware provides.
> +	 */
>  	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
>  		/*
>  		 * Writing to synthetic MSR 0x40000118 updates/changes the
> @@ -636,8 +641,12 @@ 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.
> +	 *
> +	 * HV_ACCESS_TSC_INVARIANT is always zero for the root partition. 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 2edc13ca184e..ca39044a4a60 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -549,14 +549,22 @@ 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. Skip the unnecessary
> +	 * check.

This comment needs to be changed.

	 * There is no HV_ACCESS_TSC_INVARIANT feature. Always lower the
	 * rating of 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
>