From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ting-Wei Lan <lantw44@gmail.com>
Subject: Re: Work-arounds in Xen code for Intel GFX?Re: Is:
 graphics corruption with 'xen: Support Xen pv-domains using PAT." Was:Re:
 [BUG] Characters on the screen are broken on Linux >= 3.19 with VT-d
 enabled
Date: Thu, 16 Jul 2015 23:35:34 +0800
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To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, yang.z.zhang@intel.com, kevin.tian@intel.com
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> > > > But Linux provides 'intel_iommu=igfx_off' to workaround the 
> > > > problem.
> > > > Does Xen provide similar things?
> > > 
> > > Not that I know of. The question is whether you really need VT-d, 
> > > and 
> > > if
> > > yes, why. You could still switch the iommu off for dom0 by 
> > > setting
> > > iommu=dom0-passthrough in the Xen command line (your hardware 
> > > might 
> > > not
> > > support it, though).
> > 
> > iommu=dom0-passthrough doesn't work on my hardware.
> > 
> > I found a workaround for my hardware:
> > 
> > diff --git a/xen/drivers/passthrough/vtd/quirks.c
> > b/xen/drivers/passthrough/vtd/quirks.c
> > index 69d29ab..b937ad0 100644
> > --- a/xen/drivers/passthrough/vtd/quirks.c
> > +++ b/xen/drivers/passthrough/vtd/quirks.c
> > @@ -74,6 +74,7 @@ int is_igd_vt_enabled_quirk(void)
> >  
> >      if ( !IS_ILK(ioh_id) )
> >          return 1;
> > +    return 0;
> >  
> >      /* integrated graphics on Intel platforms is located at 0:2.0 
> > */
> >      ggc = pci_conf_read16(0, 0, IGD_DEV, 0, GGC);
> 
> Lets CC the maintaners of said code.
> > 
> > 
> > This workaround is silimar to intel_iommu=igfx_off in Linux. I 
> > found
> > silimar code in function quirk_calpella_no_shadow_gtt in
> > drivers/iommu/intel-iommu.c.

As I don't know how to properly fix the problem, I made a patch to add
iommu=igfx_off option to workaround the issue in Xen.

https://gist.github.com/lantw44/9f8a94d2eeb846889a5a

Is this patch acceptable, or we should wait for a fix instead?


> > 
> > 
> > I reported the graphics corruption problems on Linux >= 3.7 here:
> > https://bugs.freedesktop.org/show_bug.cgi?id=91127

It was partially fixed by this commit (available in Linux 4.2-rc2):
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/
?id=8b572a4

But it seems it cannot be ported to Xen.


> > 
> > And here is the bug link of Xen and Linux >= 3.19 (the bug we are
> > discussing here):
> > https://bugs.freedesktop.org/show_bug.cgi?id=90037
> > 
> > I hope we can get a real fix.
> > 
> > 
> > > 
> > > 
> > > Juergen