linux-kernel - [PATCH 4.9 082/107] cpu/hotplug: detect SMT disabled by BIOS

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Date:   Tue, 14 Aug 2018 19:17:45 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Joe Mario <jmario@...hat.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: [PATCH 4.9 082/107] cpu/hotplug: detect SMT disabled by BIOS

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Josh Poimboeuf <jpoimboe@...hat.com>

commit 73d5e2b472640b1fcdb61ae8be389912ef211bda upstream

If SMT is disabled in BIOS, the CPU code doesn't properly detect it.
The /sys/devices/system/cpu/smt/control file shows 'on', and the 'l1tf'
vulnerabilities file shows SMT as vulnerable.

Fix it by forcing 'cpu_smt_control' to CPU_SMT_NOT_SUPPORTED in such a
case.  Unfortunately the detection can only be done after bringing all
the CPUs online, so we have to overwrite any previous writes to the
variable.

Reported-by: Joe Mario <jmario@...hat.com>
Tested-by: Jiri Kosina <jkosina@...e.cz>
Fixes: f048c399e0f7 ("x86/topology: Provide topology_smt_supported()")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/cpu.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2063,6 +2063,15 @@ static const struct attribute_group cpuh
 
 static int __init cpu_smt_state_init(void)
 {
+	/*
+	 * If SMT was disabled by BIOS, detect it here, after the CPUs have
+	 * been brought online.  This ensures the smt/l1tf sysfs entries are
+	 * consistent with reality.  Note this may overwrite cpu_smt_control's
+	 * previous setting.
+	 */
+	if (topology_max_smt_threads() == 1)
+		cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
+
 	return sysfs_create_group(&cpu_subsys.dev_root->kobj,
 				  &cpuhp_smt_attr_group);
 }