x86/tdx: Mark TSC reliable

x86/tdx: Mark TSC reliable

In x86 virtualization environments, including TDX, RDTSC instruction is
handled without causing a VM exit, resulting in minimal overhead and
jitters. On the other hand, other clock sources (such as HPET, ACPI
timer, APIC, etc.) necessitate VM exits to implement, resulting in more
fluctuating measurements compared to TSC. Thus, those clock sources are
not effective for calibrating TSC.

As a foundation, the host TSC is guaranteed to be invariant on any
system which enumerates TDX support.

TDX guests and the TDX module build on that foundation by enforcing:

  - Virtual TSC is monotonously incrementing for any single VCPU;
  - Virtual TSC values are consistent among all the TD’s VCPUs at the
    level supported by the CPU:
    + VMM is required to set the same TSC_ADJUST;
    + VMM must not modify from initial value of TSC_ADJUST before
      SEAMCALL;
  - The frequency is determined by TD configuration:
    + Virtual TSC frequency is specified by VMM on TDH.MNG.INIT;
    + Virtual TSC starts counting from 0 at TDH.MNG.INIT;

The result is that a reliable TSC is a TDX architectural guarantee.

Use the TSC as the only reliable clock source in TD guests, bypassing
unstable calibration.

This is similar to what the kernel already does in some VMWare and
HyperV environments.

[ dhansen: changelog tweaks ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Erdem Aktas <erdemaktas@google.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/all/20231006144549.2633-1-kirill.shutemov%40linux.intel.com