linux-kernel - Re: [PATCH] genirq: Only call irq

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Message-ID: <552E31F8.5090407@codelabs.ch>
Date:	Wed, 15 Apr 2015 11:40:08 +0200
From:	Adrian-Ken Rueegsegger <ken@...elabs.ch>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] genirq: Only call irq_ack if implemented by chip

Hi,

On 04/14/2015 08:00 PM, Thomas Gleixner wrote:
> On Tue, 14 Apr 2015, Adrian-Ken Rueegsegger wrote:
>> Restore the check if an IRQ chip implements the irq_ack function prior
>> to its invocation. Commit 22a49163e90d ("genirq: Provide compat handling
>> for chip->ack()") removed the check from handle_edge_irq while keeping
>> the check in other call paths.
> 
> How's an edge triggered interrupt without ack supposed to work?
> 
> You are missing to describe which problem you solve.

I am running Linux as a VM on top of the Muen Separation Kernel (SK)
[1], where we have implemented PCI device passthrough using VT-d. In
this case the hardware interrupt is handled by the SK/hypervisor and
injected to Linux.

To support PCI MSI(-X), we register our own platform-specific MSI
operations (x86_msi.setup_msi_irqs, etc) and implement an irq_chip that
simply provides the irq_mask and irq_unmask operations (by reusing the
"regular" mask_msi_irq/unmask_msi_irq).

After encountering a null pointer dereference due to the irq_chip not
providing an irq_ack operation, I examined the commit that changed the
source of the irq_ack call, noticing that only the check in
handle_edge_irq was dropped while the other call sites were kept. As the
commit message did not provide me with additional information for that
particular fragment of the change, I (prematurely) concluded that it was
not intentional.

If it is a prerequisite for IRQ chips to provide the irq_ack operation
when used in conjunction with handle_edge_irq, then please ignore my
patch. In that case, I will adjust our chip implementation
accordingly.

Thanks for your time,
Adrian

[1] - http://muen.codelabs.ch/
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