Subject: x86/idle: Drop incorrect smp_mb() in mwait_idle_with_hints()
From: Andrew Cooper <[email-masked]>
Date: Tue, 1 Jul 2025 18:13:27 +0100

With the recent simplifications, it becomes obvious that smp_mb() isn't the
right barrier.  Strictly speaking, MONITOR is ordered as a load, but smp_rmb()
isn't correct either, as this only pertains to local ordering.  All we need is
a compiler barrier().

Merge the barier() into the monitor() itself, along with an explantion.

No functional change.

Signed-off-by: Andrew Cooper <[email-masked]>
Acked-by: Roger Pau Monné <[email-masked]>
(cherry picked from commit e7710dd843ba9d204f6ee2973d6120c1984958a6)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index 1b316e849d6a..8de89b117aa3 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -65,8 +65,12 @@ static always_inline void monitor(
     alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR,
                       [addr] "a" (addr));
 
+    /*
+     * The memory clobber is a compiler barrier.  Subseqeunt reads from the
+     * monitored cacheline must not be reordered over MONITOR.
+     */
     asm volatile ( "monitor"
-                   :: "a" (addr), "c" (ecx), "d" (edx) );
+                   :: "a" (addr), "c" (ecx), "d" (edx) : "memory" );
 }
 
 static always_inline void mwait(unsigned int eax, unsigned int ecx)
@@ -465,7 +469,6 @@ void mwait_idle_with_hints(unsigned int eax, unsigned int ecx)
     const unsigned int *this_softirq_pending = &softirq_pending(cpu);
 
     monitor(this_softirq_pending, 0, 0);
-    smp_mb();
 
     if ( !*this_softirq_pending )
     {