Re: [Xen-devel] 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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Jul 16, 2015 at 11:35:34PM +0800, Ting-Wei Lan wrote:
> > > > > 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?

This would be the Intel VMX maintainers call. But I would just post
the patch with your Signed off By and make sure to CC xen-devel &
the VMX maintainers and see what they say.
> 
> 
> > > 
> > > 
> > > 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

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