[xen staging] x86/idle: Drop incorrect smp_mb() in mwait_idle_with_hints()

[patchbot@xxxxxxx]

commit e7710dd843ba9d204f6ee2973d6120c1984958a6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Jul 1 18:13:27 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jul 4 19:03:32 2025 +0100

    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@xxxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/acpi/cpu_idle.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index 141f0f0fac..68dd44be5b 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -66,8 +66,12 @@ 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)
@@ -453,7 +457,6 @@ 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 )
     {
--
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