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: Tuesday, July 21, 2015 2:57 PM
> 
> >>> On 21.07.15 at 02:57, <kevin.tian@xxxxxxxxx> wrote:
> >>  From: Andrew Cooper [mailto:amc96@xxxxxxxxxxxxxxxx] 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@xxxxxxxxx]
> >> >> 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@xxxxxxxxx>
> >> > 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).
> 

My point is about when i915 wants WC, then either UC (I suppose is
the case before that Linux commit) and WC (by that commit) has
no correctness problem. UC is more strict than WC. It's just performance 
difference. It's not about using WC in wrong place when it's not desired.

Thanks
Kevin

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