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

[Venkat Rao Bagalkote]

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@xxxxxxxxxxxxx>
> > Fixes: 334f3f6d7a16 ("powerpc/entry: Disable interrupts before 
> > irqentry_exit")
> > Reported-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>
> > Closes: 
> > https://lore.kernel.org/all/6f9bfb0f-b14c-468e-bb9f-c157d120d0dc@xxxxxxxxxxxxx/
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>
> > ---


This patch fixes the reported issue.

Tested-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>


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@xxxxxxxxx
> >
> >   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)