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From:   isaku.yamahata@intel.com
To:     Thomas Gleixner <tglx@linutronix.de>,
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        "H . Peter Anvin" <hpa@zytor.com>,
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Cc:     isaku.yamahata@intel.com, isaku.yamahata@gmail.com,
        Kai Huang <kai.huang@linux.intel.com>
Subject: [RFC PATCH v2 64/69] cpu/hotplug: Document that TDX also depends on booting CPUs once
Date:   Fri,  2 Jul 2021 15:05:10 -0700
Message-Id: <a934180cca67fc275659719696fff01de708565e.1625186503.git.isaku.yamahata@intel.com>
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From: Kai Huang <kai.huang@linux.intel.com>

Add a comment to explain that TDX also depends on booting logical CPUs
at least once.

TDSYSINITLP must be run on all CPUs, even software disabled CPUs in the
-nosmt case.  Fortunately, current SMT handling for #MC already supports
booting all CPUs once; the to-be-disabled sibling is booted once (and
later put into deep C-state to honor SMT=off) to allow the init code to
set CR4.MCE and avoid an unwanted shutdown on a broadcasted MCE.

Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
---
 kernel/cpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index e538518556f4..58377a03e9d6 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -446,6 +446,10 @@ static inline bool cpu_smt_allowed(unsigned int cpu)
 	 * that the init code can get a chance to set CR4.MCE on each
 	 * CPU. Otherwise, a broadcasted MCE observing CR4.MCE=0b on any
 	 * core will shutdown the machine.
+	 *
+	 * Intel TDX also requires running TDH_SYS_LP_INIT on all logical CPUs
+	 * during boot, booting all CPUs once allows TDX to play nice with
+	 * 'nosmt'.
 	 */
 	return !cpumask_test_cpu(cpu, &cpus_booted_once_mask);
 }
-- 
2.25.1