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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 21.07.15 at 02:57, <kevin.tian@intel.com> wrote:
>>  From: Andrew Cooper [mailto:amc96@hermes.cam.ac.uk] On Behalf Of Andrew 
> Cooper
>> Sent: Monday, July 20, 2015 4:21 PM
>> 
>> On 20/07/2015 02:28, Tian, Kevin wrote:
>> >> From: Ting-Wei Lan [mailto:lantw44@gmail.com]
>> >> Sent: Saturday, July 18, 2015 3:06 AM
>> >>
>> >> 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>
>> > Since igfx works before, I'd think a more proper fix should be on the
>> > bisected Linux commit or i915 to have two working correctly together.
>> > Otherwise this patch is just hiding problem.
>> 
>> The linux commit is the one which actually fixes PAT support for Linux
>> under Xen.
>> 
>> It will cause the i915 driver to actually get WC mappings when it asks
>> for them.
> 
> This is the part which I don't quite understand. WC is essentially an UC
> attribute with write buffer to accelerate the write efficiency. There 
> should be no correctness problem to use either WC or UC if i915 driver
> wants WC.

"Should" is too weak a term here: Using WC on the wrong piece of
memory or without the necessary fencing can imo very well cause
correctness problems (which would be hidden by WC -> UC
conversion behind the driver's back).

Jan