linux-kernel - Re: [PATCH 2/2] iommu/vt-d: Only remove domain when device is removed

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Message-ID: <1415290565.16601.92.camel@ul30vt.home>
Date:	Thu, 06 Nov 2014 09:16:05 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Joerg Roedel <jroedel@...e.de>
Cc:	Joerg Roedel <joro@...tes.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Myron Stowe <mstowe@...hat.com>
Subject: Re: [PATCH 2/2] iommu/vt-d: Only remove domain when device is
 removed

On Thu, 2014-11-06 at 13:54 +0100, Joerg Roedel wrote:
> Hi Alex,
> 
> On Tue, Nov 04, 2014 at 09:12:17AM -0700, Alex Williamson wrote:
> > I haven't tested it, but I'm concerned whether this has introduced a
> > domain leak.  If we think about the case of unbinding a device from a
> > host driver and attaching it to a domain through the IOMMU API, I think
> > we used to count on this path to call domain_exit(), which made the
> > domain_context_mapped() in intel_iommu_attach_device() "unlikely".  With
> > this change, isn't the test in intel_iommu_attach_device() now neither
> > likely nor unlikely and we're only removing the dev_info from the domain
> > and not destroying the domain itself?  Thanks,
> 
> As I see it, there is no leak. The DMA-API domains are kept in the
> device_domain_list and re-used when the device driver re-attaches. But
> your are right that the unlikely in intel_iommu_attach_device() isn't
> true anymore. We could probably remove it.

But the domains are unlinked from device_domain_list using
unlink_domain_info() which is called from both domain_remove_dev_info()
and domain_remove_one_dev_info() which are both part of that more
likely, unlikely branch in intel_iommu_attach_device().  So it seems
like any time we switch a device from the DMA-API to the IOMMU-API, we
lose the reference to the domain.  Is that incorrect?  I'll try to test.
Thanks,

Alex

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