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

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Date:   Thu, 7 Sep 2023 19:25:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

On Fri, Aug 25, 2023 at 5:25 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > TDX aside it might be useful to have a mechanism to select TSC over KVM
> > > clock in general.
> >
> > Sean, Paolo, any comment on this?
>
> I would expect the VMM to not advertise KVM clock if the VM is going to run on
> hosts with stable TSCs, i.e. the guest really shouldn't need to do anything in.
> But I avoid clocks and timekeeping like the plague, so take that with a grain of
> salt, e.g. maybe there's a good reason to always advertise kvmclock.

Mostly because users avoid clocks and timekeeping _even more_;
advertising kvmclock is safe in general, so userspace does it in case
the VM is later migrated to a machine with unstable TSC.

> For TDX and other paranoid guests, I assume the kernel command line is captured
> as part of attestation.   And so the existing "no-kvmclock" param should be
> sufficient to ensure the guest doesn't use KVM clock over the TSC, though IIRC
> TDX requires a constant, non-stop TSC, so it's likely not strictly necessary.

As Kirill said, the guest still has to protect itself, so the patch
makes sense (I see a v2 has been posted in the meanwhile).

Paolo