[PATCHv3] x86/tdx: Mark TSC reliable

[Kirill A. Shutemov]

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.

The host TSC is invariant on platforms where TDX is available.

In TD guests, TSC is virtualized by the TDX module, which ensures:

  - 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;

Reliable TSC is architectural guarantee for the TDX platform and it must
work for any sane TDX implementation.

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Reviewed-by: Erdem Aktas <erdemaktas@xxxxxxxxxx>
Reviewed-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Acked-by: Kai Huang <kai.huang@xxxxxxxxx>
---

 v3:
  - More details in the commit message;
  - Add Reviewed-bys and Acked-by;

 arch/x86/coco/tdx/tdx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 3e6dbd2199cf..2f27ae1e2c6b 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -816,6 +816,9 @@ void __init tdx_early_init(void)
 
 	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
 
+	/* TSC is the only reliable clock in TDX guest */
+	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
+
 	cc_vendor = CC_VENDOR_INTEL;
 	tdx_parse_tdinfo(&cc_mask);
 	cc_set_mask(cc_mask);
-- 
2.41.0