From: wei....@kernel.org <wei....@kernel.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....@kernel.org>
> ---
> Cc: Michael Kelley <mhkli...@outlook.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
