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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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