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

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Date:   Thu, 24 Aug 2023 21:31:39 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     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,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

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:
>> On 8/8/23 09:23, Kirill A. Shutemov wrote:
>> ...
>> > 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.
>> 
>> Do we need to do anything to _those_ to mark them as slightly stinky?
>
> I don't know what the rules here. As far as I can see, all other clock
> sources relevant for TDX guest have lower rating. I guess we are fine?

Ideally they are not enumerated in the first place, which prevents the
kernel from trying.

> There's notable exception to the rating order is kvmclock which is higher
> than tsc.

Which is silly aside of TDX.

> It has to be disabled, but it is not clear to me how. This topic
> is related to how we are going to filter allowed devices/drivers, so I
> would postpone the decision until we settle on wider filtering schema.

TDX aside it might be useful to have a mechanism to select TSC over KVM
clock in general.

Thanks,

        tglx