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

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Date:   Wed, 30 Aug 2023 09:33:47 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Nakajima, Jun" <jun.nakajima@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

On Tue, Aug 29 2023 at 16:01, Jun Nakajima wrote:
>> On Aug 25, 2023, at 10:09 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>> 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. :(
>
> Actually (in TDX Module 1.5 spec), the sentence is "Virtual TSC values
> are consistent among all the TD’s VCPUs at the level supported by the
> CPU, see below”.
>
> And the below:
> ---
> The host VMM is required to do the following:
> • Set up the same IA32_TSC_ADJUST values on all LPs before initializing the Intel TDX module.
> • Make sure IA32_TSC_ADJUST is not modified from its initial value before calling SEAMCALL.
>
> The Intel TDX module checks the above as part of TDH.VP.ENTER and any
> other SEAMCALL leaf function that reads TSC.

What happens when the check detects that the host modified TSC ADJUST?

What validates the VMCS TSC offset field?

> The virtualized TSC is designed to have the following characteristics:
> • The virtual TSC frequency is specified by the host VMM as an input
> to TDH.MNG.INIT in units of 25MHz – it can be between 4 and 400
> (corresponding to a range of 100MHz to 10GHz).

What validates that the frequency is correct?

How is ensured that the host does not change TSC scaling?

Thanks,

        tglx