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

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Wed, Aug 23, 2023 at 04:47:29AM +0000, Huang, Kai wrote:
> 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".

Okay, will update.

> >   - 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>

Thanks.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov