Subject: x86/idle: Convert force_mwait_ipi_wakeup to X86_BUG_MONITOR
From: Andrew Cooper <[email-masked]>
Date: Tue, 1 Jul 2025 21:40:51 +0100

We're going to want alternative-patch based on it.

No functional change.

Signed-off-by: Andrew Cooper <[email-masked]>
Acked-by: Roger Pau Monné <[email-masked]>
(cherry picked from commit b0ca0f93f47c43f8984981137af07ca3d161e3ec)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index dfa6b93070dc..4b8fb469381c 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -440,14 +440,8 @@ static int __init cf_check cpu_idle_key_init(void)
 }
 __initcall(cpu_idle_key_init);
 
-/* Force sending of a wakeup IPI regardless of mwait usage. */
-bool __ro_after_init force_mwait_ipi_wakeup;
-
 bool arch_skip_send_event_check(unsigned int cpu)
 {
-    if ( force_mwait_ipi_wakeup )
-        return false;
-
     return false;
 }
 
diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 9f8115008b67..98f1db234915 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -436,7 +436,7 @@ static void __init probe_mwait_errata(void)
     {
         printk(XENLOG_WARNING
                "Forcing IPI MWAIT wakeup due to CPU erratum\n");
-        force_mwait_ipi_wakeup = true;
+        setup_force_cpu_cap(X86_BUG_MONITOR);
     }
 }
 
diff --git a/xen/arch/x86/include/asm/cpufeatures.h b/xen/arch/x86/include/asm/cpufeatures.h
index 84c93292c80c..56231b00f15d 100644
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -53,6 +53,7 @@ XEN_CPUFEATURE(USE_VMCALL,        X86_SYNTH(30)) /* Use VMCALL instead of VMMCAL
 #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_BUG_MONITOR           X86_BUG( 5) /* MONITOR doesn't always notice writes (force IPIs) */
 
 #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. */
diff --git a/xen/arch/x86/include/asm/mwait.h b/xen/arch/x86/include/asm/mwait.h
index c52cd3f51011..000a692f6d19 100644
--- a/xen/arch/x86/include/asm/mwait.h
+++ b/xen/arch/x86/include/asm/mwait.h
@@ -13,9 +13,6 @@
 
 #define MWAIT_ECX_INTERRUPT_BREAK	0x1
 
-/* Force sending of a wakeup IPI regardless of mwait usage. */
-extern bool force_mwait_ipi_wakeup;
-
 void mwait_idle_with_hints(unsigned int eax, unsigned int ecx);
 #ifdef CONFIG_INTEL
 bool mwait_pc10_supported(void);