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

[Ting-Wei Lan <lantw44@gmail.com>]

於 一，2015-07-20 於 09:21 +0100，Andrew Cooper 提到：
> 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.

I made a new i915 bug report because the old one seems to be ignored:
https://bugs.freedesktop.org/show_bug.cgi?id=91400

> 
> > There is one possible usage to do selective IOMMU disable other 
> > than
> > global "iommu=off" switch. Then making this option general would
> > be better than igfx_off, e.g. based on BDF. But I'm not sure how it
> > is useful in reality.
> 
> It is curious that just disabling the IOMMU appears to fix the 
> problem. 

Yes, disabling IOMMU (iommu=off or iommu=igfx_off added by this patch)
avoids the problem.

A similar problem also happens on Linux >= 3.7 without using Xen, and
it can also be avoided using intel_iommu=igfx_off.

> Are there any errata you are aware of on this class of system?

With Xen:
https://bugs.freedesktop.org/show_bug.cgi?id=90037
http://lists.xenproject.org/archives/html/xen-devel/2015
-06/msg02236.html
https://bugs.freedesktop.org/show_bug.cgi?id=91400

Without Xen:
https://bugs.freedesktop.org/show_bug.cgi?id=91127

> 
> ~Andrew

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