Subject: x86/idle: Fix buggy "x86/mwait-idle: enable interrupts before C1 on Xeons"
From: Andrew Cooper <[email-masked]>
Date: Wed, 2 Jul 2025 14:51:38 +0100

The check of this_softirq_pending must be performed with irqs disabled, but
this property was broken by an attempt to optimise entry/exit latency.

Commit c227233ad64c in Linux (which we copied into Xen) was fixed up by
edc8fc01f608 in Linux, which we have so far missed.

Going to sleep without waking on interrupts is nonsensical outside of
play_dead(), so overload this to select between two possible MWAITs, the
second using the STI shadow to cover MWAIT for exactly the same reason as we
do in safe_halt().

Fixes: b17e0ec72ede ("x86/mwait-idle: enable interrupts before C1 on Xeons")
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>
(cherry picked from commit 9b0f0f6e235618c2764e925b58c4bfe412730ced)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index b876c7781eef..0b7e7636bc0c 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -79,6 +79,13 @@ static always_inline void mwait(unsigned int eax, unsigned int ecx)
                    :: "a" (eax), "c" (ecx) );
 }
 
+static always_inline void sti_mwait_cli(unsigned int eax, unsigned int ecx)
+{
+    /* STI shadow covers MWAIT. */
+    asm volatile ( "sti; mwait; cli"
+                   :: "a" (eax), "c" (ecx) );
+}
+
 #define GET_HW_RES_IN_NS(msr, val) \
     do { rdmsrl(msr, val); val = tsc_ticks2ns(val); } while( 0 )
 #define GET_MC6_RES(val)  GET_HW_RES_IN_NS(0x664, val)
@@ -473,12 +480,19 @@ void mwait_idle_with_hints(unsigned int eax, unsigned int ecx)
 
     monitor(this_softirq_pending, 0, 0);
 
+    ASSERT(!local_irq_is_enabled());
+
     if ( !*this_softirq_pending )
     {
         struct cpu_info *info = get_cpu_info();
 
         spec_ctrl_enter_idle(info);
-        mwait(eax, ecx);
+
+        if ( ecx & MWAIT_ECX_INTERRUPT_BREAK )
+            mwait(eax, ecx);
+        else
+            sti_mwait_cli(eax, ecx);
+
         spec_ctrl_exit_idle(info);
     }
 
diff --git a/xen/arch/x86/cpu/mwait-idle.c b/xen/arch/x86/cpu/mwait-idle.c
index 12c36257b7be..ad421d8bb76a 100644
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -973,12 +973,8 @@ static void cf_check mwait_idle(void)
 
 	update_last_cx_stat(power, cx, before);
 
-	if (cx->irq_enable_early)
-		local_irq_enable();
-
-	mwait_idle_with_hints(cx->address, MWAIT_ECX_INTERRUPT_BREAK);
-
-	local_irq_disable();
+	mwait_idle_with_hints(cx->address,
+			      cx->irq_enable_early ? 0 : MWAIT_ECX_INTERRUPT_BREAK);
 
 	after = alternative_call(cpuidle_get_tick);