[xen staging] x86/idle: Fix buggy "x86/mwait-idle: enable interrupts before C1 on Xeons"

[patchbot@xxxxxxx]

commit 9b0f0f6e235618c2764e925b58c4bfe412730ced
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Jul 2 14:51:38 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jul 4 19:03:32 2025 +0100

    x86/idle: Fix buggy "x86/mwait-idle: enable interrupts before C1 on Xeons"
    
    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 <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/acpi/cpu_idle.c  | 16 +++++++++++++++-
 xen/arch/x86/cpu/mwait-idle.c |  8 ++------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index c5d7a6c6fe..e50a9234a0 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -80,6 +80,13 @@ 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)
@@ -461,12 +468,19 @@ 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 5c16f5ad3a..5e98011bfd 100644
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -946,12 +946,8 @@ 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);
 
--
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