[tip: x86/tdx] x86/tdx: Mark TSC reliable

[tip-bot2 for Kirill A. Shutemov]

The following commit has been merged into the x86/tdx branch of tip:

Commit-ID:     9ee4318c157b9802589b746cc340bae3142d984c
Gitweb:        https://git.kernel.org/tip/9ee4318c157b9802589b746cc340bae3142d984c
Author:        Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
AuthorDate:    Fri, 06 Oct 2023 17:45:49 +03:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Fri, 06 Oct 2023 10:00:04 -07:00

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@xxxxxxxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@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>
Link: https://lore.kernel.org/all/20231006144549.2633-1-kirill.shutemov%40linux.intel.com
---
 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 3e6dbd2..2f27ae1 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);