Re: [PATCH 2/2] KVM: PPC: Replay pending interrupts before entering the guest

[Mukesh Kumar Chaurasiya]

On Fri, Oct 30, 2026 at 10:00:50PM +0530, Ritesh Harjani wrote:
> 
> Hi Shrikanth,
> 
> Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> writes:
> 
> > After applying preempt disable patch, i.e PATCH 1/2, Christian reported
> > a subsequent warning stopping his board to boot properly.
> >
> > WARNING: at .kvmppc_fix_ee_before_entry+0x10/0x28, CPU#0: qemu-system-ppc/4667
> > CPU: 0 UID: 1000 PID: 4667 Comm: qemu-system-ppc Tainted: G        W           7.3.0-rc4-2-powerpc64-smp #1 PREEMPT
> > Tainted: [W]=WARN
> > Hardware name: varisys,CYRUS5040 e5500 0x80240012 CoreNet Generic
> > NIP [c00000000005dc3c] .kvmppc_fix_ee_before_entry+0x10/0x28
> > LR [c00000000005f298] .kvmppc_handle_exit+0x5b4/0x5e8
> > Call Trace:
> > [c000000086587780] [c00000000005ee10] .kvmppc_handle_exit+0x12c/0x5e8 (unreliable)
> > [c000000086587830] [c000000000062068] .kvmppc_resume_host+0xb8/0x10c
> >
> > It triggers below warning...
> >
> > static inline void kvmppc_fix_ee_before_entry(void)
> > {
> >         trace_hardirqs_on();
> >
> >         /*
> >          * To avoid races, the caller must have gone directly from having
> >          * interrupts fully-enabled to hard-disabled.
> >          */
> >         WARN_ON(local_paca->irq_happened != PACA_IRQ_HARD_DIS);
> >
> > This happens since kvmppc_prepare_to_enter does first local_irq_disable
> > followed by hard_irq_disable. This leaves a small window where interrupt
> > may occur and it could set the irq pending bit in PACA. When that
> > happens replay that interrupt before entering the guest.
> >
> > Fixes: 12013e3d4695 ("KVM: powerpc: Use generic xfer to guest work function")
> > Reported-by: Christian Zigotzky <chzigotzky@xxxxxxxxxxx>
> > Closes: https://lore.kernel.org/all/b8f82519-9ae5-c247-e020-e79c627e57df@xxxxxxxxxxx/
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >  arch/powerpc/kvm/powerpc.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index 9194cf492d1c..847ff07c364b 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -147,6 +147,19 @@ int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
> >  			continue;
> >  		}
> >  
> > +#ifdef CONFIG_PPC64
> > +		/*
> > +		 * Interrupt arrived between the soft and hard
> > +		 * disable. Replay it and retry guest entry.
> > +		 */
> > +		if (unlikely(local_paca->irq_happened != PACA_IRQ_HARD_DIS)) {
> > +			local_irq_enable();
> > +			local_irq_disable();
> > +			hard_irq_disable();
> > +			continue;
> > +		}
> > +#endif
> > +
> 
> I looked at the commit which causes this regression, I see there are few
> more call sites which can hit the same WARN in kvmppc_fix_ee_before_entry().
> 
> So what I was thinking was - while we are at it - do you think we could
> use the following helper instead of {local|hard}_irq_{disable|enable}()
> pair and ...
> 
> 
> /*
>  * Hard disable interrupts before entering the guest.
>  *
>  * On 64-bit don't use local_irq_disable() here. It only soft-masks, so
>  * an interrupt can still come in before we hard disable. That interrupt
>  * gets marked pending in irq_happened and kvmppc_fix_ee_before_entry()
>  * then throws it away. hard_irq_disable() clears MSR[EE] first and does
>  * the irq tracing itself, so there's no such window.
>  *
>  * On 32-bit there is no soft mask and hard_irq_disable() doesn't do the
>  * irq tracing, so use local_irq_disable().
>  */
> static inline void kvmppc_irq_disable_for_entry(void)
> {
> #ifdef CONFIG_PPC64
>         hard_irq_disable();
> #else
>         local_irq_disable();
> #endif
> }
> 
> ... and then update the other call sites too which the following commit
> added - which can cause the WARN.
>    Fixes: 12013e3d4695 ("KVM: powerpc: Use generic xfer to guest work function")
> 
> Thoughts?
> 
Yeah, i also feel this is better to have.
Personally i don't want this ifdef all around the code, It's better to
have it in one place.
There's one more case like this here[1].

[1]https://lore.kernel.org/all/a14571be1897955f43d9994f0e093de3ada6ecde.1793447880.git.ritesh.list@xxxxxxxxx/

Regards,
Mukesh
> -ritesh
> 
> >  		guest_enter_irqoff();
> >  		return 1;
> >  	}
> > -- 
> > 2.52.0