Re: [PATCH] x86/tdx: Mark TSC reliable

[Thomas Gleixner <tglx@linutronix.de>]

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