linux-kernel - [tip: x86/core] x86/smp: Use dedicated cache-line for mwait_play

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Message-ID: <168726605135.404.14181848725041591301.tip-bot2@tip-bot2>
Date:   Tue, 20 Jun 2023 13:00:51 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ashok Raj <ashok.raj@...el.com>,
        "Borislav Petkov (AMD)" <bp@...en8.de>, stable@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86/smp: Use dedicated cache-line for mwait_play_dead()

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     f9c9987bf52f4e42e940ae217333ebb5a4c3b506
Gitweb:        https://git.kernel.org/tip/f9c9987bf52f4e42e940ae217333ebb5a4c3b506
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Thu, 15 Jun 2023 22:33:55 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Tue, 20 Jun 2023 14:51:47 +02:00

x86/smp: Use dedicated cache-line for mwait_play_dead()

Monitoring idletask::thread_info::flags in mwait_play_dead() has been an
obvious choice as all what is needed is a cache line which is not written
by other CPUs.

But there is a use case where a "dead" CPU needs to be brought out of
MWAIT: kexec().

This is required as kexec() can overwrite text, pagetables, stacks and the
monitored cacheline of the original kernel. The latter causes MWAIT to
resume execution which obviously causes havoc on the kexec kernel which
results usually in triple faults.

Use a dedicated per CPU storage to prepare for that.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Ashok Raj <ashok.raj@...el.com>
Reviewed-by: Borislav Petkov (AMD) <bp@...en8.de>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20230615193330.434553750@linutronix.de

---
 arch/x86/kernel/smpboot.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 352f0ce..c5ac5d7 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -101,6 +101,17 @@ EXPORT_PER_CPU_SYMBOL(cpu_die_map);
 DEFINE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);
 EXPORT_PER_CPU_SYMBOL(cpu_info);
 
+struct mwait_cpu_dead {
+	unsigned int	control;
+	unsigned int	status;
+};
+
+/*
+ * Cache line aligned data for mwait_play_dead(). Separate on purpose so
+ * that it's unlikely to be touched by other CPUs.
+ */
+static DEFINE_PER_CPU_ALIGNED(struct mwait_cpu_dead, mwait_cpu_dead);
+
 /* Logical package management. We might want to allocate that dynamically */
 unsigned int __max_logical_packages __read_mostly;
 EXPORT_SYMBOL(__max_logical_packages);
@@ -1758,10 +1769,10 @@ EXPORT_SYMBOL_GPL(cond_wakeup_cpu0);
  */
 static inline void mwait_play_dead(void)
 {
+	struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
 	unsigned int eax, ebx, ecx, edx;
 	unsigned int highest_cstate = 0;
 	unsigned int highest_subcstate = 0;
-	void *mwait_ptr;
 	int i;
 
 	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
@@ -1796,13 +1807,6 @@ static inline void mwait_play_dead(void)
 			(highest_subcstate - 1);
 	}
 
-	/*
-	 * This should be a memory location in a cache line which is
-	 * unlikely to be touched by other processors.  The actual
-	 * content is immaterial as it is not actually modified in any way.
-	 */
-	mwait_ptr = &current_thread_info()->flags;
-
 	wbinvd();
 
 	while (1) {
@@ -1814,9 +1818,9 @@ static inline void mwait_play_dead(void)
 		 * case where we return around the loop.
 		 */
 		mb();
-		clflush(mwait_ptr);
+		clflush(md);
 		mb();
-		__monitor(mwait_ptr, 0, 0);
+		__monitor(md, 0, 0);
 		mb();
 		__mwait(eax, 0);