linux-kernel - Re: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus

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Date:	Sat, 12 Nov 2011 00:51:48 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	linux-pci@...r.kernel.org, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] intel-iommu: Default to non-coherent for domains
 unattached to iommus

On Fri, 2011-11-11 at 16:45 -0800, Chris Wright wrote:
> > So if you were to ditch the whole idea of a per-domain runtime update,
> > and instead calculate a global value for 'iommu_coherency' at boot time,
> > by iterating over for_each_active_iommu()¹, I think that would be a
> > better way to deal with the issue. And you *could* really call that a
> > 'fix'.
> > 
> > Make sense?
> 
> Ideally, yes.  Not sure we can practically do it though.  Would have to
> be sure we force incoherent access mode for the busted hw. 

Why's it not practical? You do the *same* loop we currently have in
domain_update_iommu_coherency(), except that you do it just *once*, over
all active IOMMUs, at boot time. And then you just use that result
forever more.

So if *any* IOMMU in the system is non-coherent, you run them all that
way.

-- 
dwmw2

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