[PATCH 5/6] x86/idle: Drop incorrect smp_mb() in mwait_idle_with_hints()

[Andrew Cooper]

With the recent simplifications, it becomes obvious that smp_mb() isn't the
right barrier; all we need is a compiler barrier.

Include this in monitor() itself, along with an explantion.

No functional change.

Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
---
CC: Jan Beulich <jbeul...@suse.com>
CC: Roger Pau Monné <roger....@citrix.com>
CC: Anthony PERARD <anthony.per...@vates.tech>
CC: Michal Orzel <michal.or...@amd.com>
CC: Julien Grall <jul...@xen.org>
CC: Stefano Stabellini <sstabell...@kernel.org>
---
 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 13040ef467a0..de020dfee87f 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 hoisted 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)
@@ -456,7 +460,6 @@ void mwait_idle_with_hints(unsigned int eax, unsigned int 
ecx)
                    [in_mwait] "=m" (stat->in_mwait));
 
     monitor(this_softirq_pending, 0, 0);
-    smp_mb();
 
     if ( !*this_softirq_pending )
     {
-- 
2.39.5