linux-kernel - [PATCH v2] Revert "x86/smp: Eliminate mwait_play_dead_cpuid

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <12674167.O9o76ZdvQC@rjwysocki.net>
Date: Thu, 29 May 2025 15:40:43 +0200
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: x86 Maintainers <x86@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>,
 Len Brown <lenb@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>, Ingo Molnar <mingo@...hat.com>,
 Todd Brandt <todd.e.brandt@...ux.intel.com>
Subject: [PATCH v2] Revert "x86/smp: Eliminate mwait_play_dead_cpuid_hint()"

From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>

Revert commit 96040f7273e2 ("x86/smp: Eliminate mwait_play_dead_cpuid_hint()")
because it introduced a significant power regression on systems that
start with "nosmt" in the kernel command line.

Namely, on such systems, SMT siblings permanently go offline early,
when cpuidle has not been initialized yet, so after the above commit,
hlt_play_dead() is called for them.  Later on, when the processor
attempts to enter a deep package C-state, including PC10 which is
requisite for reaching minimum power in suspend-to-idle, it is not
able to do that because of the SMT siblings staying in C1 (which
they have been put into by HLT).

As a result, the idle power (including power in suspend-to-idle)
rises quite dramatically on those systems with all of the possible
consequences, which (needless to say) may not be expected by their
users.

This issue is hard to debug and potentially dangerous, so it needs to
be addressed as soon as possible in a way that will work for 6.15.y,
hence the revert.

Of course, after this revert, the issue that commit 96040f7273e2
attempted to address will be back and it will need to be fixed again
later.

Fixes: 96040f7273e2 ("x86/smp: Eliminate mwait_play_dead_cpuid_hint()")
Reported-by: Todd Brandt <todd.e.brandt@...ux.intel.com>
Tested-by: Todd Brandt <todd.e.brandt@...ux.intel.com>
Cc: 6.15+ <stable@...r.kernel.org> # 6.15+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---

This supersedes https://lore.kernel.org/linux-pm/7811828.EvYhyI6sBW@rjwysocki.net/

v1 -> v2:
   * Send as a standalone patch.
   * Extend the changelog.

I honestly don't think that there is any reasonable alternative to this
revert that would be suitable for 6.15.y (y > 0), so I'm going to apply
it and include it in a PR during the remaining part of this merge window
unless somebody beats me to this.

Thanks!

---
 arch/x86/kernel/smpboot.c |   54 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 47 insertions(+), 7 deletions(-)

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1238,10 +1238,6 @@
 	local_irq_disable();
 }

-/*
- * We need to flush the caches before going to sleep, lest we have
- * dirty data in our caches when we come back up.
- */
 void __noreturn mwait_play_dead(unsigned int eax_hint)
 {
 	struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
@@ -1288,6 +1284,50 @@
 }

 /*
+ * We need to flush the caches before going to sleep, lest we have
+ * dirty data in our caches when we come back up.
+ */
+static inline void mwait_play_dead_cpuid_hint(void)
+{
+	unsigned int eax, ebx, ecx, edx;
+	unsigned int highest_cstate = 0;
+	unsigned int highest_subcstate = 0;
+	int i;
+
+	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
+	    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
+		return;
+	if (!this_cpu_has(X86_FEATURE_MWAIT))
+		return;
+	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
+		return;
+
+	eax = CPUID_LEAF_MWAIT;
+	ecx = 0;
+	native_cpuid(&eax, &ebx, &ecx, &edx);
+
+	/*
+	 * eax will be 0 if EDX enumeration is not valid.
+	 * Initialized below to cstate, sub_cstate value when EDX is valid.
+	 */
+	if (!(ecx & CPUID5_ECX_EXTENSIONS_SUPPORTED)) {
+		eax = 0;
+	} else {
+		edx >>= MWAIT_SUBSTATE_SIZE;
+		for (i = 0; i < 7 && edx; i++, edx >>= MWAIT_SUBSTATE_SIZE) {
+			if (edx & MWAIT_SUBSTATE_MASK) {
+				highest_cstate = i;
+				highest_subcstate = edx & MWAIT_SUBSTATE_MASK;
+			}
+		}
+		eax = (highest_cstate << MWAIT_SUBSTATE_SIZE) |
+			(highest_subcstate - 1);
+	}
+
+	mwait_play_dead(eax);
+}
+
+/*
  * Kick all "offline" CPUs out of mwait on kexec(). See comment in
  * mwait_play_dead().
  */
@@ -1337,9 +1377,9 @@
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);

-	/* Below returns only on error. */
-	cpuidle_play_dead();
-	hlt_play_dead();
+	mwait_play_dead_cpuid_hint();
+	if (cpuidle_play_dead())
+		hlt_play_dead();
 }

 #else /* ... !CONFIG_HOTPLUG_CPU */