File xsa471-10.patch of Package xen.39862

# Commit e7710dd843ba9d204f6ee2973d6120c1984958a6
# Date 2025-07-04 19:03:32 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/idle: Drop incorrect smp_mb() in mwait_idle_with_hints()

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 <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -64,8 +64,12 @@ 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 ( ".byte 0x0f, 0x01, 0xc8" /* 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)
@@ -442,7 +446,6 @@ void mwait_idle_with_hints(unsigned int
     const unsigned int *this_softirq_pending = &softirq_pending(cpu);
 
     monitor(this_softirq_pending, 0, 0);
-    smp_mb();
 
     if ( !*this_softirq_pending )
     {