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From: Thomas Gleixner <tglx@linutronix.de>
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bp@alien8.de>,
 Andy Lutomirski <luto@kernel.org>, Kuppuswamy Sathyanarayanan
 <sathyanarayanan.kuppuswamy@linux.intel.com>, Elena Reshetova
 <elena.reshetova@intel.com>, Jun Nakajima <jun.nakajima@intel.com>,
 x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable
In-Reply-To: <20230825135226.ikxvskplvppghwtz@box.shutemov.name>
References: <20230808162320.27297-1-kirill.shutemov@linux.intel.com>
 <ecc11d54-6aaa-f755-9436-ae15b94fb627@intel.com>
 <20230808200111.nz74tmschph435ri@box> <87bkewcufi.ffs@tglx>
 <20230825135226.ikxvskplvppghwtz@box.shutemov.name>
Date: Fri, 25 Aug 2023 19:09:41 +0200
Message-ID: <87pm3baw16.ffs@tglx>
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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.
>>=20
>> Nice weasel wording. What's the definition of "small"?
>
> The newer spec says "Virtual TSC values are consistent among all the TD=
=E2=80=99s
> VCPUs at the level supported by the CPU".

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