From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, November 12, 2024 10:18 AM
>
> Enable X86_FEATURE_TSC_RELIABLE by default as X86_FEATURE_TSC_RELIABLE is
> independent from invariant TSC and should have never been gated by the
> HV_ACCESS_TSC_INVARIANT privilege.
I think originally X86_FEATURE_TSC_RELIABLE was gated by the Hyper-V
TSC Invariant feature because otherwise VM live migration may cause
the TSC value reported by the RDTSC/RDTSCP instruction in the guest
to abruptly change frequency and value. In such cases, the TSC isn't
useable by the kernel or user space.
Enabling the Hyper-V TSC Invariant feature fixes that by using the
hardware scaling available in more recent processors to automatically
fixup the TSC value returned by RDTSC/RDTSCP in the guest.
Is there a practical problem that is fixed by always enabling
X86_FEATURE_TSC_RELIABLE?
Michael
>
> To elaborate, the HV_ACCESS_TSC_INVARIANT privilege allows certain types of
> guests to opt-in to invariant TSC by writing the
> HV_X64_MSR_TSC_INVARIANT_CONTROL register. Not all guests will have this
> privilege and the hypervisor will automatically opt-in certain types of
> guests (e.g. EXO partitions) to invariant TSC, but this functionality is
> unrelated to the TSC reliability.
>
> Signed-off-by: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
> ---
> arch/x86/kernel/cpu/mshyperv.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index d18078834ded..14412afcc398 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -515,7 +515,7 @@ static void __init ms_hyperv_init_platform(void)
> machine_ops.crash_shutdown = hv_machine_crash_shutdown;
> #endif
> #endif
> - if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> + if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT)
> /*
> * Writing to synthetic MSR 0x40000118 updates/changes the
> * guest visible CPUIDs. Setting bit 0 of this MSR enables
> @@ -526,8 +526,8 @@ static void __init ms_hyperv_init_platform(void)
> * is called.
> */
> wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, HV_EXPOSE_INVARIANT_TSC);
> - setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> - }
> +
> + setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
>
> /*
> * Generation 2 instances don't support reading the NMI status from
>
>
