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

[藍挺瑋 <lantw44@gmail.com>]

於 二，2015-06-16 於 11:06 +0200，Juergen Gross 提到：
> On 06/16/2015 10:55 AM, Ting-Wei Lan wrote:
> > Juergen Gross 於 西元2015年06月16日 12:30 寫道:
> > > On 06/15/2015 09:03 PM, Ting-Wei Lan wrote:
> > > > 於 一，2015-06-15 於 14:55 -0400，Konrad Rzeszutek Wilk 提到：
> > > > > On Sat, Jun 13, 2015 at 12:43:14AM +0800, Ting-Wei Lan wrote:
> > > > > > When using Linux >= 3.19 as the dom0 kernel, characters on 
> > > > > > the
> > > > > > screen become
> > > > > > broken after the graphic driver is loaded. The commit that 
> > > > > > breaks
> > > > > > it is
> > > > > > (found by git bisect):
> > > > > > 
> > > > > > 
> > > > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux
> > > > > > .git/com
> > > > > > mit/?id=47591df
> > > > > 
> > > > > Lets CC Juergen
> > > > > > 
> > > > > > 
> > > > > > Screenshot when the system run in single user mode:
> > > > > > 
> > > > > >    https://bugs.freedesktop.org/attachment.cgi?id=115079
> > > 
> > > Are those messages to be expected:
> > > 
> > > (XEN) Scrubbing Free RAM on 1 nodes using 2 CPUs
> > > (XEN) [VT-D]DMAR:[DMA Write] Request device [0000:00:02.0] fault 
> > > addr
> > > b61eef000, iommu reg = ffff82c000203000
> > > (XEN) [VT-D]DMAR: reason 05 - PTE Write access is not set
> > > (XEN)
> > > .................................................................
> > > .done.
> > > 
> > > I'm not familiar with VT-D internals, but seeing these messages 
> > > for the
> > > video device during RAM scrubbing makes wonder if everything is 
> > > correct
> > > regarding the VT-D and memory setup...
> ...
> > > > > > I still remember that there was a similar problem found two 
> > > > > > years
> > > > > > ago on the
> > > > > > same hardware with similar broken screen output and it also 
> > > > > > crashed
> > > > > > after
> > > > > > Xorg was started, but I cannot confirm that they are the 
> > > > > > same
> > > > > > problems. I
> > > > > > don't know whether error messages are simliar.
> > > > > > 
> > > > > > The old problem happens on Linux 3.7 ~ 3.10 with VT-d 
> > > > > > enabled. It
> > > > > > only
> > > > > > happened when not using Xen, so I added 'intel_iommu=off' 
> > > > > > to Linux
> > > > > > boot
> > > > > > arguments to workaround it.
> > > 
> > > Hmm, do you see any chance in finding the commit which made it 
> > > working
> > > again? Perhaps there was some workaround for this hardware which 
> > > is
> > > missing in Xen now...
> > 
> > After some tests, I found the information I provided before was
> > incorrect. It seems the problem happens on all Linux >= 3.7, 
> > including
> > Linux 4.0.5, so the old problem was never fixed. Here are some 
> > 'dmesg |
> > grep -i iommu' outputs.
> 
> So a Xen-specific error is rather improbable, correct?

But Linux provides 'intel_iommu=igfx_off' to workaround the problem.
Does Xen provide similar things?

> 
> I'd continue with sending the new information to the Intel graphics
> team.

I am going to reopen DRM/Intel bug 90037 on freedesktop bugzilla.

> 
> 
> Juergen

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