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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, July 20, 2015 8:24 PM
> 
> >>> 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).

kind of agree here. IIRC, to have i915 working correctly in Dom0, user
needs to manually turn on CONFIG_DMAR and CONFIG_INTEL_IOMMU
although there's no IOMMU exposed. Otherwise i915 driver will use plain 
virt_to_phys when programming GTT table which causes trouble. 
Recently there are some improvements in this part, but still some IOMMU
specific tricks remain. So having the options default on under Xen looks
better.

Thanks
Kevin

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