linux-kernel - Re: [PATCH v11 09/10] genirq/msi: map/unmap the MSI doorbells on msi_domain_alloc/free

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Message-ID: <f8e83628-5fb0-70d4-c32c-237d190ec98f@redhat.com>
Date:	Tue, 26 Jul 2016 11:54:55 +0200
From:	Auger Eric <eric.auger@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	eric.auger.pro@...il.com, Marc Zyngier <marc.zyngier@....com>,
	christoffer.dall@...aro.org, andre.przywara@....com,
	robin.murphy@....com, alex.williamson@...hat.com,
	will.deacon@....com, joro@...tes.org,
	Jason Cooper <jason@...edaemon.net>,
	LAK <linux-arm-kernel@...ts.infradead.org>, drjones@...hat.com,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	pbonzini@...hat.com, LKML <linux-kernel@...r.kernel.org>,
	Bharat.Bhushan@...escale.com, pranav.sawargaonkar@...il.com,
	p.fedin@...sung.com, iommu@...ts.linux-foundation.org,
	Jean-Philippe.Brucker@....com, yehuday@...vell.com,
	Manish.Jaggi@...iumnetworks.com, robert.richter@...iumnetworks.com,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v11 09/10] genirq/msi: map/unmap the MSI doorbells on
 msi_domain_alloc/free_irqs

Hi Thomas,

On 26/07/2016 11:00, Thomas Gleixner wrote:
> B1;2802;0cEric,
> 
> On Mon, 25 Jul 2016, Auger Eric wrote:
>> On 20/07/2016 11:04, Thomas Gleixner wrote:
>>> On Tue, 19 Jul 2016, Eric Auger wrote:
>>>> +		if (ret) {
>>>> +			for (; i >= 0; i--) {
>>>> +				struct irq_data *d = irq_get_irq_data(virq + i);
>>>> +
>>>> +				msi_handle_doorbell_mappings(d, false);
>>>> +			}
>>>> +			irq_domain_free_irqs(virq, desc->nvec_used);
>>>> +			desc->irq = 0;
>>>> +			goto error;
>>>
>>> How is that supposed to work? You clear desc->irq and then you call
>>> ops->handle_error.
>>
>> if I don't clear the desc->irq I enter an infinite loop in
>> pci_enable_msix_range.
>>
>> This happens because msix_capability_init and pcie_enable_msix returns 1.
>> In msix_capability_init, at out_avail: we enumerate the msi_desc which have
>> a non zero irq, hence the returned value equal to 1.
>>
>> Currently the only handle_error ops I found, pci_msi_domain_handle_error
>> does not use irq field so works although questionable.
> 
> The logic here is: If the allocation does not succeed for the requested number
> of interrupts, we tell the caller how many interrupts we were able to set up.
> So the caller can decide what to do.
> 
> In your case you don't want to have a partial allocation, so instead of
> playing silly games with desc->irq you should add a flag which tells the PCI
> code that you are not interested in a partial allocation and that it should
> return an error code instead.

In that case can we consider we even succeeded in allocating 1 MSI? In case the
IOMMU mapping fails, the MSI transaction will never reach the target MSI frame
so it is not usable. So when you mean "partial" I understand we did not succeed
in allocating maxvec IRQs, correct? Here we succeeded in allocating 0 IRQ and still
msi_capability_init returns 1.

msi_capability_init doc-comment says "a positive return value indicates the number of
interrupts which could have been allocated."

I understand allocation success currently only depends on the fact virq was allocated
and set to desc->irq. But with that IOMMU stuff doesn't the criteria changes?


> Something like PCI_DEV_FLAGS_MSI_NO_PARTIAL_ALLOC should do the trick.
> 
>> As for the irq_domain_free_irqs I think I can remove it since handled later.
> 
> Not only the free_irqs(). You should let the teardown function handle
> everything including your doorbell mapping teardown. It's nothing special and
> free_msi_irqs() at the end of msix_capability_init() will take care of it.
Yep I was forced to call free_irqs myself since free_msi_irqs was doing nothing
due the fact I resetted the irq field. Wrong thing loop ;-)

Thanks

Eric

> 
> Thanks,
> 
> 	tglx
>