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

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Message-ID: <1415292194.16601.96.camel@ul30vt.home>
Date:	Thu, 06 Nov 2014 09:43:14 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Joerg Roedel <jroedel@...e.de>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	Myron Stowe <mstowe@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>
Subject: Re: [PATCH 2/2] iommu/vt-d: Only remove domain when device is
 removed

On Thu, 2014-11-06 at 09:16 -0700, Alex Williamson wrote:
> 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.

Trying the simple approach, a printk in each of alloc_domain() and
free_domain_mem(), this is what I see when I start and stop a VM with an
assigned device:

alloc_domain(): ffff8801e22ac000
free_domain_mem(ffff8801e22ac000)
alloc_domain(): ffff8801e3425c80

The IOMMU API domain is alloc'd and free'd, then a new DMA-API domain is
alloc'd.  There are no frees of the DMA-API domain.  Thanks,

Alex

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