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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 8/20/26 7:17 PM, Mukesh Kumar Chaurasiya (IBM) wrote:
> 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
> 

Indeed. Thanks for the fix and good explanation.

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>

> 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)