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

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Message-ID: <20230825135226.ikxvskplvppghwtz@box.shutemov.name>
Date:   Fri, 25 Aug 2023 16:52:26 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

On Thu, Aug 24, 2023 at 05:49:05PM +0200, Thomas Gleixner wrote:
> On Tue, Aug 08 2023 at 23:01, Kirill A. Shutemov wrote:
> > On Tue, Aug 08, 2023 at 10:13:05AM -0700, Dave Hansen wrote:
> >> I take it this is carved in stone in the TDX specs somewhere.  A
> >> reference would be nice.
> >
> > TDX Module 1.0 spec:
> >
> > 	5.3.5. Time Stamp Counter (TSC)
> >
> > 	TDX provides a trusted virtual TSC to the guest TDs. TSC value is
> > 	monotonously incrementing, starting from 0 on TD initialization by the
> > 	host VMM. The deviation between virtual TSC values read by each VCPU is
> > 	small.
> 
> Nice weasel wording. What's the definition of "small"?

The newer spec says "Virtual TSC values are consistent among all the TD’s
VCPUs at the level supported by the CPU".

-- 
  Kiryl Shutsemau / Kirill A. Shutemov