Re: [Xen-devel] [PATCH] VT-d: add iommu=igfx_off option to workaround graphics issues

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.07.15 at 14:12, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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@xxxxxxxxx>
> 
> 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.

Perhaps it should just assume an IOMMU is in use when running under
Xen. Having inspected all those code places quite some time ago, I
came to the conclusion that making this assumption is better than
the current one of there not being an enabled IOMMU (and I adjusted
our kernels accordingly).

> 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).

The presence of a DMAR table is not a 1:1 representation of whether
Xen has the IOMMU enabled.

Jan


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