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

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Message-ID: <CAAYXXYyuykiz80t4VO1ShdPGh=T3z0tcsV4EGG3N-rP9+4PFnA@mail.gmail.com>
Date:   Tue, 22 Aug 2023 16:39:54 -0700
From:   Erdem Aktas <erdemaktas@...gle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "Reshetova, Elena" <elena.reshetova@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable

Reviewed-by: Erdem Aktas <erdemaktas@...gle.com>

Thanks Kirill for this patch.  I think this patch is necessary to
prevent guest marking TSC as unreliable due to the possible
calibration failures at runtime. We also tested it.




On Tue, Aug 8, 2023 at 11:14 PM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Wed, Aug 09, 2023 at 05:44:37AM +0000, Reshetova, Elena wrote:
> > >
> > > 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?
> >
> > What about acpi_pm?
> > See this:
> > https://github.com/intel/tdx/commit/045692772ab4ef75062a83cc6e4ffa22cab40226
>
> clocksource_acpi_pm.rating is 200 while TSC is 300.
>
> > > There's notable exception to the rating order is kvmclock which is higher
> > > than tsc. 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.
> >
> > One option is to include "no-kvmclock" into kernel command line, which
> > is attested. Another option is to try to disable it explicitly, like we had
> > in past:
> > https://github.com/intel/tdx/commit/6b0357f2115c1bdd158c0c8836f4f541517bf375
> >
> > The obvious issues with command line is that it is going to 1) grow
> > considerably if we try to disable everything we can via command line
> > and 2) there is a high chance that in practice people will not use secure default
> > and/or forget to verify the correct status of cmd line. But this is to be
> > expected I guess for any security method that involves attestation unfortunately.
>
> I guess command line is fine, until we have coherent solution on
> filtering.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov