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

[Venkat Rao Bagalkote <venkat88@linux.ibm.com>]

On 20/08/26 8:17 pm, Shrikanth Hegde wrote:
>
>
> 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>
>> ---


This patch fixes the reported issue.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>


Regards,

Venkat.


>> 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)
>
>