linux-kernel - Re: [PATCH 1/4] iommu: Add iommu_device_group callback and iommu

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Date:	Thu, 1 Dec 2011 11:57:25 +1100
From:	David Gibson <dwg@....ibm.com>
To:	Chris Wright <chrisw@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	joerg.roedel@....com, dwmw2@...radead.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	agraf@...e.de, scottwood@...escale.com, B08248@...escale.com
Subject: Re: [PATCH 1/4] iommu: Add iommu_device_group callback and
 iommu_group sysfs entry

On Wed, Nov 30, 2011 at 04:52:20PM -0800, Chris Wright wrote:
> * David Gibson (dwg@....ibm.com) wrote:
> > On Tue, Nov 29, 2011 at 10:25:51PM -0700, Alex Williamson wrote:
> > > On Wed, 2011-11-30 at 15:51 +1100, Benjamin Herrenschmidt wrote:
> > > > On Wed, 2011-11-30 at 13:42 +1100, David Gibson wrote:
> > > > 
> > > > > > +static ssize_t show_iommu_group(struct device *dev,
> > > > > > +				struct device_attribute *attr, char *buf)
> > > > > > +{
> > > > > > +	unsigned int groupid;
> > > > > > +
> > > > > > +	if (iommu_device_group(dev, &groupid))
> > > > > > +		return 0;
> > > > > > +
> > > > > > +	return sprintf(buf, "%u", groupid);
> > > > > > +}
> > > > > > +static DEVICE_ATTR(iommu_group, S_IRUGO, show_iommu_group, NULL);
> > > > > 
> > > > > Hrm.  Assuming the group is is an unsigned int seems dangerous to me.
> > > > > More seriously, we really want these to be unique across the whole
> > > > > system, but they're allocated by the iommu driver which can't
> > > > > guarantee that if it's not the only one present.  Seems to me it would
> > > > > be safer to have an actual iommu_group structure allocated for each
> > > > > group, and use the pointer to it as the ID to hand around (with NULL
> > > > > meaning "no iommu" / untranslated).  The structure could contain a
> > > > > more human readable - or more relevant to platform documentation - ID
> > > > > where appropriate.
> > > 
> > > Note that iommu drivers are registered per bus_type, so the unique pair
> > > is {bus_type, groupid}, which seems sufficient for vfio.
> > 
> > Hrm.  That's.. far from obvious.  And still breaks down if we have two
> > separate iommus on the same bus type (e.g. two independent PCI host
> > bridges with inbuilt IOMMUs).
> 
> Happens to still work for Intel IOMMU on x86 the way Alex wrote the
> Intel VT-d patch in this series, as well as AMD IOMMU.  The caveat for
> AMD IOMMU is that the groupid generation would break (as-is) once
> there's support for multiple PCI segments.  This is not an inherent
> shortcoming of the groupid mechanism though, just a current limitation
> of AMD IOMMU's implementation.  Alex overloaded B:D.F for those which is
> a convenient id since that maps to the device (or in the case of devices
> behind a PCIe-to-PCI bridge, the requestor ID of all devices behind the
> bridge, or "the group").

"Happens to still work" is not exactly a ringing endorsement.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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