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

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Message-ID: <20230825134746.k7hkpa3e7wnsuq7m@box.shutemov.name>
Date:   Fri, 25 Aug 2023 16:47:46 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

On Thu, Aug 24, 2023 at 09:31:39PM +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:
> >> 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.

We can ask QEMU/KVM not to advertise them to TDX guest, but guest has to
protect itself as the VMM is not trusted. And we are back to device
filtering...

> > 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.

Sean, Paolo, any comment on this?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov