Re: [PATCHv2] x86/tdx: Mark TSC reliable

[Huang, Kai]

On Wed, 2023-08-23 at 02:18 +0300, Kirill A. Shutemov wrote:
> 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.
> 
> In TD guests, TSC is virtualized by the TDX module, which ensures:
> 
>   - Virtual TSC values are consistent among all the TD’s VCPUs;
>   - Monotonously incrementing for any single VCPU;

Nit:

    - Virtual TSC is monotonously incrementing ...

Otherwise here "incrementing" looks like a noun, which means you need to use
"monotonous" rather than "monotonously".


>   - The frequency is determined by TD configuration. The host TSC is
>     invariant on platforms where TDX is available.
> 
> 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>

Acked-by: Kai Huang <kai.huang@xxxxxxxxx>