[PATCH V2] powerpc/entry: Fix irq_soft_mask corruption on replayed interrupt exit

["Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>]

When __replay_soft_interrupts() replays a pending interrupt (e.g.
PACA_IRQ_DEC → timer_interrupt), it calls the handler directly with a
synthetic pt_regs. The DEFINE_INTERRUPT_HANDLER_ASYNC wrapper around
each handler calls arch_interrupt_async_exit_prepare() on the way out,
which calls arch_interrupt_exit_prepare() → local_irq_disable() →
arch_local_irq_disable(), which does:

    irq_soft_mask_set(IRQS_DISABLED)   /* 0x1 */

This unconditionally overwrites irq_soft_mask with IRQS_DISABLED (0x1),
stripping the IRQS_PMI_DISABLED (0x2) bit. The result is that
irq_soft_mask is 0x1 instead of IRQS_ALL_DISABLED (0x3) when the
handler returns to __replay_soft_interrupts().

For a normally-taken interrupt this is harmless: the next interrupt
always enters through arch_interrupt_enter_prepare() which
unconditionally sets irq_soft_mask to IRQS_ALL_DISABLED.
But during replay, next_interrupt() is called
directly between replayed handlers without going back through
arch_interrupt_enter_prepare(), so the stripped bit is never restored.
next_interrupt() then fires a WARNING:

    WARNING: arch/powerpc/kernel/irq_64.c:75
    WARN_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED)

This was introduced by commit bee25f97ad24 ("powerpc: Enable
GENERIC_ENTRY feature"). Before that commit, the old
interrupt_async_exit_prepare() called irq_exit() followed by an empty
interrupt_exit_prepare() stub and never touched irq_soft_mask at all,
so the soft mask was left at IRQS_ALL_DISABLED throughout replay.

The root cause: arch_interrupt_exit_prepare() uses local_irq_disable()
whose only job is to set the IRQS_DISABLED bit; it has no knowledge of
IRQS_PMI_DISABLED. It is there to satisfy irqentry_exit()'s
requirement that interrupts be disabled, but using the plain
irq_soft_mask_set(IRQS_ALL_DISABLED) is the right primitive:

  - irq_soft_mask_set(IRQS_ALL_DISABLED): sets soft mask to 0x3
    (both IRQS_DISABLED and IRQS_PMI_DISABLED). Touches only the soft
    mask. MSR[EE] and PACA_IRQ_HARD_DIS are already correct because
    hard interrupts were never re-enabled during replay
    (PACA_IRQ_REPLAYING is in PACA_IRQ_MUST_HARD_MASK, which blocks
    should_hard_irq_enable()).

  - local_irq_disable() / arch_local_irq_disable(): sets soft mask to
    IRQS_DISABLED (0x1) only, silently dropping IRQS_PMI_DISABLED.

  - hard_irq_disable(): also issues __mtmsrd to clear MSR[EE] in
    hardware and sets PACA_IRQ_HARD_DIS — redundant and wrong here
    since both are already set.

Fix by replacing local_irq_disable() with irq_soft_mask_set(IRQS_ALL_DISABLED)
in arch_interrupt_exit_prepare(), making the exit symmetric with the
entry path in arch_interrupt_enter_prepare() which always sets
IRQS_ALL_DISABLED.

The warning was observed early in boot on a POWER10 pseries guest
during kmem_cache_init_late(), where a spinlock release triggers
interrupt replay that processes a pending timer interrupt.

Debugger state confirming the bug:
  Before timer_interrupt(&regs):
    irq_soft_mask = 0x3 (IRQS_ALL_DISABLED)   correct
    irq_happened  = 0x41 (HARD_DIS|REPLAYING)  correct
  After timer_interrupt(&regs) returns:
    irq_soft_mask = 0x1 (IRQS_DISABLED)        WRONG — PMI bit stripped
    irq_happened  = 0x41                        unchanged

Fixes: 334f3f6d7a16 ("powerpc/entry: Disable interrupts before irqentry_exit")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/all/6f9bfb0f-b14c-468e-bb9f-c157d120d0dc@linux.ibm.com/
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
Change log:
V1 -> V2:
	- Instead of using hard_irq_disable use irq_soft_mask_set
V1: https://lore.kernel.org/all/20260812152035.1661781-1-mkchauras@gmail.com

 arch/powerpc/include/asm/entry-common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/entry-common.h b/arch/powerpc/include/asm/entry-common.h
index c5adb5006361..2a153dca962c 100644
--- a/arch/powerpc/include/asm/entry-common.h
+++ b/arch/powerpc/include/asm/entry-common.h
@@ -270,7 +270,7 @@ static inline void arch_interrupt_exit_prepare(struct pt_regs *regs)
 	}
 
 	/* irqentry_exit expects to be called with interrupts disabled */
-	local_irq_disable();
+	irq_soft_mask_set(IRQS_ALL_DISABLED);
 }
 
 static inline void arch_interrupt_async_enter_prepare(struct pt_regs *regs)
-- 
2.55.0