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

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: "Reshetova, Elena" <elena.reshetova@intel.com>
Cc: "Hansen, Dave" <dave.hansen@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	"Lutomirski, Andy" <luto@kernel.org>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"Nakajima, Jun" <jun.nakajima@intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/tdx: Mark TSC reliable
Date: Wed, 9 Aug 2023 09:13:48 +0300	[thread overview]
Message-ID: <20230809061348.vhm5ie4uzystwfya@box.shutemov.name> (raw)
In-Reply-To: <DM8PR11MB5750DEFF988068EFAFE28667E712A@DM8PR11MB5750.namprd11.prod.outlook.com>

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