linux-kernel - Re: [PATCHv2] x86/tdx: Mark TSC reliable

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Message-ID: <f5dc2de9d3997c3a89bdedee6cb0bf554f400ccc.camel@intel.com>
Date:   Wed, 23 Aug 2023 04:47:29 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>
CC:     "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCHv2] x86/tdx: Mark TSC reliable

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@...ux.intel.com>

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