From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Wed, 2 Apr 2025 17:24:58 +0000 (+0100)
Subject: x86/idle: Remove MFENCEs for X86_BUG_CLFLUSH_MONITOR in mwait_idle_with_hints() and... 
X-Git-Tag: v6.16-rc1~195^2~26^2~18
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1f13c60d84e880df6698441026e64f84c7110c49;p=thirdparty%2Flinux.git

x86/idle: Remove MFENCEs for X86_BUG_CLFLUSH_MONITOR in mwait_idle_with_hints() and prefer_mwait_c1_over_halt()

The following commit, 12 years ago:

  7e98b7192046 ("x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers")

added barriers around the CLFLUSH in mwait_idle_with_hints(), justified with:

  ... and add memory barriers around it since the documentation is explicit
  that CLFLUSH is only ordered with respect to MFENCE.

This also triggered, 11 years ago, the same adjustment in:

  f8e617f45829 ("sched/idle/x86: Optimize unnecessary mwait_idle() resched IPIs")

during development, although it failed to get the static_cpu_has_bug() treatment.

X86_BUG_CLFLUSH_MONITOR (a.k.a the AAI65 errata) is specific to Intel CPUs,
and the SDM currently states:

  Executions of the CLFLUSH instruction are ordered with respect to each
  other and with respect to writes, locked read-modify-write instructions,
  and fence instructions[1].

With footnote 1 reading:

  Earlier versions of this manual specified that executions of the CLFLUSH
  instruction were ordered only by the MFENCE instruction.  All processors
  implementing the CLFLUSH instruction also order it relative to the other
  operations enumerated above.

i.e. The SDM was incorrect at the time, and barriers should not have been
inserted.  Double checking the original AAI65 errata (not available from
intel.com any more) shows no mention of barriers either.

Note: If this were a general codepath, the MFENCEs would be needed, because
      AMD CPUs of the same vintage do sport otherwise-unordered CLFLUSHs.

Remove the unnecessary barriers. Furthermore, use a plain alternative(),
rather than static_cpu_has_bug() and/or no optimisation.  The workaround
is a single instruction.

Use an explicit %rax pointer rather than a general memory operand, because
MONITOR takes the pointer implicitly in the same way.

[ mingo: Cleaned up the commit a bit. ]

Fixes: 7e98b7192046 ("x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20250402172458.1378112-1-andrew.cooper3@citrix.com
---

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index ce857ef54cf15..54dc313bcdf01 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -116,13 +116,10 @@ static __always_inline void __sti_mwait(unsigned long eax, unsigned long ecx)
 static __always_inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 {
 	if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
-		if (static_cpu_has_bug(X86_BUG_CLFLUSH_MONITOR)) {
-			mb();
-			clflush((void *)&current_thread_info()->flags);
-			mb();
-		}
+		const void *addr = &current_thread_info()->flags;
 
-		__monitor((void *)&current_thread_info()->flags, 0, 0);
+		alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
+		__monitor(addr, 0, 0);
 
 		if (!need_resched()) {
 			if (ecx & 1) {
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 91f6ff6188528..bda47d93c8f91 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -907,13 +907,10 @@ static __init bool prefer_mwait_c1_over_halt(void)
 static __cpuidle void mwait_idle(void)
 {
 	if (!current_set_polling_and_test()) {
-		if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
-			mb(); /* quirk */
-			clflush((void *)&current_thread_info()->flags);
-			mb(); /* quirk */
-		}
+		const void *addr = &current_thread_info()->flags;
 
-		__monitor((void *)&current_thread_info()->flags, 0, 0);
+		alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
+		__monitor(addr, 0, 0);
 		if (!need_resched()) {
 			__sti_mwait(0, 0);
 			raw_local_irq_disable();