From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH] VT-d: add iommu=igfx_off option to
 workaround graphics issues
Date: Mon, 20 Jul 2015 13:12:01 +0100
Message-ID: <55ACE591.5000809@citrix.com>
References: <1437159952-4134-1-git-send-email-lantw44@gmail.com>
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To: Ting-Wei Lan <lantw44@gmail.com>, xen-devel@lists.xen.org
Cc: Yang Zhang <yang.z.zhang@intel.com>, Kevin Tian <kevin.tian@intel.com>, Jan Beulich <jbeulich@suse.com>
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On 17/07/15 20:05, Ting-Wei Lan wrote:
> When using Linux >= 3.19 (commit 47591df) as dom0 on some Intel Ironlake
> devices, It is possible to encounter graphics issues that make screen
> unreadable or crash the system. It was reported in freedesktop bugzilla:
>
> https://bugs.freedesktop.org/show_bug.cgi?id=90037
>
> As we still cannot find a proper fix for this problem, this patch adds
> iommu=igfx_off option that is similar to Linux intel_iommu=igfx_off for
> users to manually workaround the problem.
>
> Signed-off-by: Ting-Wei Lan <lantw44@gmail.com>

Having looked into this issue, the i915 driver has several workarounds
in it for systems when the IOMMU is in use.  In some cases there are
plain errata, while in other cases there are specific hardware features
which don't function if the IOMMU is enabled.

In all cases this is gated on Linux's idea of whether the IOMMU is
enabled.  When used under Xen, Linux has no clue that the IOMMU exists,
or that Xen has turned it on.

Longterm, we should probably see about not clobbering the DMAR table and
making dom0 aware that there is an IOMMU.  (I still don't actually know
why we have this asymmetry between Intel and AMD).

However in the short term, having a command line workaround like this
seems appropriate.  It at least allows the user to tell Xen not to play
with the IOMMU behind an unsuspecting dom0.

~Andrew