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

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Message-ID: <87pm3baw16.ffs@tglx>
Date:   Fri, 25 Aug 2023 19:09:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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 Fri, Aug 25 2023 at 16:52, Kirill A. Shutemov wrote:
> 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".

That means what? It's not a guarantee for consistency either. :(