linux-kernel - Re: [PATCH v11 10/10] genirq/msi: use the MSI doorbell's IOVA when requested

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Date:	Tue, 26 Jul 2016 11:04:18 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Auger Eric <eric.auger@...hat.com>
cc:	eric.auger.pro@...il.com, marc.zyngier@....com,
	christoffer.dall@...aro.org, andre.przywara@....com,
	robin.murphy@....com, alex.williamson@...hat.com,
	will.deacon@....com, joro@...tes.org, jason@...edaemon.net,
	linux-arm-kernel@...ts.infradead.org, drjones@...hat.com,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	pbonzini@...hat.com, 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
Subject: Re: [PATCH v11 10/10] genirq/msi: use the MSI doorbell's IOVA when
 requested

Eric,

On Mon, 25 Jul 2016, Auger Eric wrote:
> On 20/07/2016 11:09, Thomas Gleixner wrote:
> > On Tue, 19 Jul 2016, Eric Auger wrote:
> >> @@ -63,10 +63,18 @@ static int msi_compose(struct irq_data *irq_data,
> >>  {
> >>  	int ret = 0;
> >>  
> >> -	if (erase)
> >> +	if (erase) {
> >>  		memset(msg, 0, sizeof(*msg));
> >> -	else
> >> +	} else {
> >> +		struct device *dev;
> >> +
> >>  		ret = irq_chip_compose_msi_msg(irq_data, msg);
> >> +		if (ret)
> >> +			return ret;
> >> +
> >> +		dev = msi_desc_to_dev(irq_data_get_msi_desc(irq_data));
> >> +		WARN_ON(iommu_msi_msg_pa_to_va(dev, msg));
> > 
> > What the heck is this call doing? And why is there only a WARN_ON and not a
> > proper error return code handling?
> 
> iommu_msi_msg_pa_to_va is part of the new iommu-msi API introduced in PART I
> of this series. This helper function detects the physical address found in
> the MSI message has a corresponding allocated IOVA. This happens if the MSI
> doorbell is accessed through an IOMMU and this IOMMU do not bypass the MSI
> addresses (ARM case). Allocation of this IOVA was performed in the previous
> patch.
>
> So, if this is the case, the physical address is swapped with the IOVA
> address. That way the PCIe device will send the MSI with this IOVA and
> the address will be translated by the IOMMU into the target MSI doorbell PA.
> 
> Hope this clarifies

No, it does not. You are explaining in great length what that function is
doing, but you are not explaining WHY your don't do a proper return code
handling and just do a WARN_ON() and happily proceed. If that function fails
then the interrupt will not be functional, so WHY on earth are you continuing?

Thanks,

	tglx