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Date:   Fri, 19 Jun 2020 09:40:53 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Daniel Gutson <daniel@eclypsium.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "David S. Miller" <davem@davemloft.net>,
        Rob Herring <robh@kernel.org>, Tony Luck <tony.luck@intel.com>,
        Rahul Tanwar <rahul.tanwar@linux.intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-kernel@vger.kernel.org, Richard Hughes <hughsient@gmail.com>
Subject: Re: [PATCH] Ability to read the MKTME status from userspace
Message-ID: <20200619074053.GA32683@zn.tnic>
References: <20200618210215.23602-1-daniel.gutson@eclypsium.com>
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On Thu, Jun 18, 2020 at 07:34:21PM -0300, Daniel Gutson wrote:
> Besides being a CPU capability, it has to be enabled by the BIOS, which is
> what the flag represents.

... yes, and if it is disabled in the BIOS, you clear the CPU cap flag.
Something like this untested diff:

---
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index c25a67a34bd3..59d8342f6a64 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -525,6 +525,7 @@ static void detect_tme(struct cpuinfo_x86 *c)
 	if (!TME_ACTIVATE_LOCKED(tme_activate) || !TME_ACTIVATE_ENABLED(tme_activate)) {
 		pr_info_once("x86/tme: not enabled by BIOS\n");
 		mktme_status = MKTME_DISABLED;
+		clear_cpu_cap(c, X86_FEATURE_TME);
 		return;
 	}
 
@@ -553,10 +554,11 @@ static void detect_tme(struct cpuinfo_x86 *c)
 		pr_info_once("x86/mktme: disabled by BIOS\n");
 	}
 
-	if (mktme_status == MKTME_UNINITIALIZED) {
-		/* MKTME is usable */
+	/* MKTME is usable */
+	if (mktme_status == MKTME_UNINITIALIZED)
 		mktme_status = MKTME_ENABLED;
-	}
+	else if (mktme_status == MKTME_DISABLED)
+		clear_cpu_cap(c, X86_FEATURE_TME);
 
 	/*
 	 * KeyID bits effectively lower the number of physical address

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette