linux-kernel - Re: [PATCH] x86/cpuinfo: Clear X86_FEATURE

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Date:   Wed, 15 Jun 2022 12:54:25 -0700
From:   Alison Schofield <alison.schofield@...el.com>
To:     Richard Hughes <hughsient@...il.com>
Cc:     Martin Fernandez <martin.fernandez@...ypsium.com>,
        linux-kernel@...r.kernel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, mingo@...hat.com,
        tglx@...utronix.de, daniel.gutson@...ypsium.com,
        alex.bazhaniuk@...ypsium.com
Subject: Re: [PATCH] x86/cpuinfo: Clear X86_FEATURE_TME if TME/MKTME is
 disabled by BIOS

On Wed, Jun 15, 2022 at 08:34:58PM +0100, Richard Hughes wrote:
> On Wed, 15 Jun 2022 at 20:06, Alison Schofield
> <alison.schofield@...el.com> wrote:
> > My first reaction is lying about the cpuinfo is not a soln, since
> > it creates a problem for a users currently relying on cpuinfo to be
> > the source of truth for TME.
> 
> I think you have to qualify "source of truth". At the moment the CPU
> reports "Yes! I support TME!" and then for one reason or another the
> platform turns it off and actually there's no memory encryption of
> your secrets at all. There's seemingly no userspace way of telling if
> TME is actually active. We were told that we shouldn't export the
> "platform has disabled a CPU feature" in sysfs and just to clear the
> cpuid flag that gets exported (like AMD is currently doing) which is
> what Martin proposed here. Programs want to know the true CPU
> capability can do __get_cpuid_count() like they can for the SME/SEV
> capabilities.
>
Disagree on sending folks to use __get_cpuid_count() when they already
have cpuinfo.

Why is a sysfs entry TME-enabled 0/1 a bad thing?  It can be documented
to have the same meaning as the log message.

You keep referring to AMD. How is their exception documented?

Alison

> > Are we to tell them to go look in the
> > log now, because fwupd folks didn't want to ;)
> 
> We're not telling anyone to use the log; grepping megabytes of
> unformatted kernel logs is a terrible (and slow) way to get one
> boolean value.
> 
> Richard.