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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.07.15 at 16:12, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 20/07/15 14:55, Jan Beulich wrote:
>>>>> On 20.07.15 at 14:34, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 20/07/15 13:24, Jan Beulich wrote:
>>>>>>> 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).
>>> In at least one case, an errata workaround involves issuing extra IOMMU
>>> commands.  We cannot safely let even dom0 perform this.
>> Mind pointing out that one case? In our Xen kernels, IOMMU code
>> gets compiled out, so it is impossible for the driver to issue extra
>> IOMMU commands...
> 
> But a distro is going to want to ship a single kernel, especially with
> PVops these days.

Right, but you see that there continue to be examples of what isn't
being taken care of in pv-ops.

Jan


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