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

From: Thomas Gleixner <tglx@linutronix.de>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>
Cc: 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, Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable
Date: Thu, 24 Aug 2023 21:31:39 +0200	[thread overview]
Message-ID: <878ra0ck4k.ffs@tglx> (raw)
In-Reply-To: <20230808200111.nz74tmschph435ri@box>

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