linux-kernel - Re: [PATCH v2] ARM: KVM: add irqfd and irq routing support

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Date:	Thu, 19 Jun 2014 16:40:20 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Will Deacon <will.deacon@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>
CC:	"eric.auger@...com" <eric.auger@...com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"christophe.barnichon@...com" <christophe.barnichon@...com>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] ARM: KVM: add irqfd and irq routing support

On 06/19/2014 04:13 PM, Will Deacon wrote:
> Hi all,
> 
> I'm currently adding VFIO support for kvmtool, so I'm interested in this
> patch series (although actually from a PCI perspective).
> 
> Eric: can you CC me on future versions of this series please? Once things
> start to stabilise, I can help with testing.

Hi Will,

sure I will CC you.
> 
> On Thu, Jun 05, 2014 at 03:39:50PM +0100, Christoffer Dall wrote:
>> On Thu, Jun 05, 2014 at 03:15:15PM +0200, Eric Auger wrote:
>>> On 06/05/2014 12:28 PM, Christoffer Dall wrote:
>>>> On Mon, Jun 02, 2014 at 09:29:56AM +0200, Eric Auger wrote:
>>>>> +                  kvm_debug("Inject irqchip routed vIRQ %d\n",
>>>>> +                                  e->irqchip.pin);
>>>>> +                  kvm_vgic_inject_irq(kvm, 0, spi, level);
>>>>> +                  /*
>>>>> +                   * toggling down vIRQ wire is directly handled in
>>>>> +                   * process_maintenance for this reason:
>>>>> +                   * irqfd_resampler_ack is called in
>>>>> +                   * process_maintenance which holds the dist lock.
>>>>> +                   * irqfd_resampler_ack calls kvm_set_irq
>>>>> +                   * which ends_up calling kvm_vgic_inject_irq.
>>>>> +                   * This later attempts to take the lock -> deadlock!
>>>>> +                   */
>>>>
>>>> Not sure I understand this comment.  What are we trying to achieve, are
>>>> we using some sort of a workaround to avoid a deadlock?
>>>
>>> What I wanted to point out here is I would have prefered to handle both
>>> levels 0 and 1 in a symetrical manner. irqfd_resampler_ack (in eventfd)
>>> is calling kvm_set_irq with level 0. This would be the prefered way to
>>> toggle down the SPI at GIC input instead of doing this in
>>> process_maintenance in a dirty manner. However this does work because
>>> irqfd_resampler_ack is called in process_maintenance (the place where
>>> the EOI is analyzed). process_maintenance holds the dist lock and would
>>> eventually call kvm_vgic_inject_irq which also attempts to take the lock.
>>>
>>
>> I'm afraid that's too much of a hack.  There's an external mechanism to
>> set an interrupt line to active (level=1) or inactive (level=0) and we
>> must support both.
>>
>> The fact that vgic_process_maintenance() can set the interrupt line to
>> inactive is just something we exploit to properly handle level-triggered
>> interrupts, but the main API to the VGIC must absolutely be supported.
>>
>> Am I completely wrong here?
>>
>> The locking issue can be solved by splitting up the locking into a finer
>> granularity as needed or deferring the call to irqfd_resampler_ack()
>> until after unlocking the distributor lock in kvm_vgic_sync_hwstate().
> 
> Why can't we do what PowerPC does for mpic and x86 does for IOAPIC and
> simply drop the distributor lock across the call to kvm_notify_acked_irq?

Yes, I am about to release a new version for this RFC that uses a finer
granularity for the dist lock, as you and Christoffer suggested.

> 
> Given that I think the eventfd callbacks can block, holding a spinlock isn't
> safe anyway, regardless of the vgic re-entrancy issue.
yes you're fully right.

Best Regards

Eric
> 
> Will
> 

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