Subject: x86/idle: Remove MFENCEs for CLFLUSH_MONITOR
From: Andrew Cooper <[email-masked]>
Date: Tue, 1 Apr 2025 15:55:29 +0100

Commit 48d32458bcd4 ("x86, idle: add barriers to CLFLUSH workaround") was
inherited from Linux and added MFENCEs around the AAI65 errata fix.

The SDM now 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 MFENCEs came about because of an incorrect statement in the SDM.

The Spec Update (no longer available on Intel's website) simply says "issue a
CLFLUSH", with no mention of MFENCEs.

As this erratum is specific to Intel, it's fine to remove the the MFENCEs; AMD
CPUs of a similar vintage do sport otherwise-unordered CLFLUSHs.

Move the feature bit into the BUG range (rather than FEATURE), and move the
workaround into monitor() itself.

The erratum check itself must use setup_force_cpu_cap().  It needs activating
if any CPU needs it, not if all of them need it.

Fixes: 48d32458bcd4 ("x86, idle: add barriers to CLFLUSH workaround")
Fixes: 96d1b237ae9b ("x86/Intel: work around Xeon 7400 series erratum AAI65")
Link: https://web.archive.org/web/20090219054841/http://download.intel.com/design/xeon/specupdt/32033601.pdf
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>
(cherry picked from commit f77ef3443542a2c2bbd59ee66178287d4fa5b43f)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index ec2d570dc27b..f1b9e2fbf6e7 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -62,6 +62,9 @@
 static always_inline void monitor(
     const void *addr, unsigned int ecx, unsigned int edx)
 {
+    alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR,
+                      [addr] "a" (addr));
+
     asm volatile ( "monitor"
                    :: "a" (addr), "c" (ecx), "d" (edx) );
 }
@@ -488,13 +491,6 @@ void mwait_idle_with_hints(unsigned int eax, unsigned int ecx)
     s_time_t expires = per_cpu(timer_deadline, cpu);
     const void *monitor_addr = &mwait_wakeup(cpu);
 
-    if ( boot_cpu_has(X86_FEATURE_CLFLUSH_MONITOR) )
-    {
-        mb();
-        clflush(monitor_addr);
-        mb();
-    }
-
     monitor(monitor_addr, 0, 0);
     smp_mb();
 
@@ -929,19 +925,7 @@ void cf_check acpi_dead_idle(void)
 
         while ( 1 )
         {
-            /*
-             * 1. The CLFLUSH is a workaround for erratum AAI65 for
-             * the Xeon 7400 series.  
-             * 2. The WBINVD is insufficient due to the spurious-wakeup
-             * case where we return around the loop.
-             * 3. Unlike wbinvd, clflush is a light weight but not serializing 
-             * instruction, hence memory fence is necessary to make sure all 
-             * load/store visible before flush cache line.
-             */
-            mb();
-            clflush(mwait_ptr);
             monitor(mwait_ptr, 0, 0);
-            mb();
             mwait(cx->address, 0);
         }
     }
diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 6f2b3fffdd34..04a002e1e0c9 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -446,6 +446,7 @@ static void __init probe_mwait_errata(void)
  *
  * Xeon 7400 erratum AAI65 (and further newer Xeons)
  * MONITOR/MWAIT may have excessive false wakeups
+ * https://web.archive.org/web/20090219054841/http://download.intel.com/design/xeon/specupdt/32033601.pdf
  */
 static void Intel_errata_workarounds(struct cpuinfo_x86 *c)
 {
@@ -463,7 +464,7 @@ static void Intel_errata_workarounds(struct cpuinfo_x86 *c)
 
 	if (c->x86 == 6 && cpu_has_clflush &&
 	    (c->x86_model == 29 || c->x86_model == 46 || c->x86_model == 47))
-		__set_bit(X86_FEATURE_CLFLUSH_MONITOR, c->x86_capability);
+		setup_force_cpu_cap(X86_BUG_CLFLUSH_MONITOR);
 
 	probe_c3_errata(c);
 	if (system_state < SYS_STATE_smp_boot)
diff --git a/xen/arch/x86/include/asm/cpufeatures.h b/xen/arch/x86/include/asm/cpufeatures.h
index 9e3ed21c026d..84c93292c80c 100644
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -19,7 +19,7 @@ XEN_CPUFEATURE(ARCH_PERFMON,      X86_SYNTH( 3)) /* Intel Architectural PerfMon
 XEN_CPUFEATURE(TSC_RELIABLE,      X86_SYNTH( 4)) /* TSC is known to be reliable */
 XEN_CPUFEATURE(XTOPOLOGY,         X86_SYNTH( 5)) /* cpu topology enum extensions */
 XEN_CPUFEATURE(CPUID_FAULTING,    X86_SYNTH( 6)) /* cpuid faulting */
-XEN_CPUFEATURE(CLFLUSH_MONITOR,   X86_SYNTH( 7)) /* clflush reqd with monitor */
+/* Bit 7 unused */
 XEN_CPUFEATURE(APERFMPERF,        X86_SYNTH( 8)) /* APERFMPERF */
 XEN_CPUFEATURE(MFENCE_RDTSC,      X86_SYNTH( 9)) /* MFENCE synchronizes RDTSC */
 XEN_CPUFEATURE(XEN_SMEP,          X86_SYNTH(10)) /* SMEP gets used by Xen itself */
@@ -52,6 +52,7 @@ XEN_CPUFEATURE(USE_VMCALL,        X86_SYNTH(30)) /* Use VMCALL instead of VMMCAL
 #define X86_BUG_NULL_SEG          X86_BUG( 1) /* NULL-ing a selector preserves the base and limit. */
 #define X86_BUG_CLFLUSH_MFENCE    X86_BUG( 2) /* MFENCE needed to serialise CLFLUSH */
 #define X86_BUG_IBPB_NO_RET       X86_BUG( 3) /* IBPB doesn't flush the RSB/RAS */
+#define X86_BUG_CLFLUSH_MONITOR   X86_BUG( 4) /* MONITOR requires CLFLUSH */
 
 #define X86_SPEC_NO_LFENCE_ENTRY_PV X86_BUG(16) /* (No) safety LFENCE for SPEC_CTRL_ENTRY_PV. */
 #define X86_SPEC_NO_LFENCE_ENTRY_INTR X86_BUG(17) /* (No) safety LFENCE for SPEC_CTRL_ENTRY_INTR. */