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* [PATCH V3] powerpc/entry: Fix irq_soft_mask corruption on replayed interrupt exit
@ 2026-09-04  9:08 Mukesh Kumar Chaurasiya (IBM)
  2026-09-04  9:11 ` Mukesh Kumar Chaurasiya
  0 siblings, 1 reply; 2+ messages in thread
From: Mukesh Kumar Chaurasiya (IBM) @ 2026-09-04  9:08 UTC (permalink / raw)
  To: maddy, mpe, npiggin, chleroy, sshegde, mkchauras, mchauras,
	ritesh.list, linuxppc-dev, linux-kernel
  Cc: Venkat Rao Bagalkote

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)

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

The fix is to replace local_irq_disable() with hard_irq_disable().

hard_irq_disable() is the right primitive here for two reasons:

1. On PPC64 (hw_irq.h:301) it calls irq_soft_mask_set_return(IRQS_ALL_DISABLED),
   setting the soft mask to 0x3 (both IRQS_DISABLED and IRQS_PMI_DISABLED),
   which preserves the PMI bit and fixes the WARNING. The additional
   work it does (__hard_irq_disable(), PACA_IRQ_HARD_DIS |=) is
   redundant but safe since both are already set at this point in the
   exit path; the trace_hardirqs_off() inside is guarded by
   if (!arch_irqs_disabled_flags(flags)) so it will not double-fire.

2. On PPC32 (hw_irq.h:467) hard_irq_disable() maps to
   arch_local_irq_disable() -> __hard_irq_disable(), which clears
   MSR[EE] in hardware. This is exactly correct: PPC32 has no soft-mask
   PACA mechanism, so the hardware disable is the right way to satisfy
   irqentry_exit()'s requirement. This also fixes a build error on PPC32
   where irq_soft_mask_set() is only defined under CONFIG_PPC64:

       arch/powerpc/include/asm/entry-common.h:273: error: implicit
       declaration of function 'irq_soft_mask_set'

   Using hard_irq_disable() requires no #ifdef and is consistent with
   how the rest of the entry code (e.g. entry-common.h:463) handles the
   same PPC32/PPC64 split.

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/
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
Change log:
V2 -> V3:
	- Revert to hard_irq_disable
	- Removed some dead code
V2: https://lore.kernel.org/all/20260820134718.2176411-1-mkchauras@gmail.com
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 94083516df57..80b07750b531 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();
+	hard_irq_disable();
 }
 
 static inline void arch_interrupt_async_enter_prepare(struct pt_regs *regs)
-- 
2.55.0


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2026-09-04  9:08 [PATCH V3] powerpc/entry: Fix irq_soft_mask corruption on replayed interrupt exit Mukesh Kumar Chaurasiya (IBM)
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