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

[Ting-Wei Lan <lantw44@xxxxxxxxx>]

Jan Beulich æ èå2015å07æ21æ 15:33 åé:
> > > > On 21.07.15 at 09:23, <kevin.tian@xxxxxxxxx> wrote:
> > >   From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > > Sent: Tuesday, July 21, 2015 3:17 PM
> > > > > > On 21.07.15 at 09:05, <kevin.tian@xxxxxxxxx> wrote:
> > > > >   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
> > > > > > 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.
> > > In this you assume there are no misguided attempts to request
> > > WC in the driver, which would have gone unnoticed as long as WC
> > > didn't become the effective attribute. And "misguided" here is
> > > meant to include cases where hardware errata may need taking
> > > care of.
> > I don't understand this point. If it's misguided attempts it'd be
> > same on bare metal.
> Not if bare metal has a workaround in place depending on (or even
> implemented in) IOMMU code. Also iirc the reported said that a
> similar problem existed (exists?) on native too.
On 3.7 <= Linux < 4.2-rc2 without Xen, the screen output is broken, and 
the system crashes after display server is started.
On Linux >= 4.2-rc2 without Xen, the screen output is normal because 
this patch is added
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8b572a4
but these messages are showed every a few seconds. If I keep using it, 
it will crash in about two hours.
DMAR: DMAR:[DMA Write] Request device [00:02.0] fault addr fdf70000
DMAR:[fault reason 05] PTE Write access is not set

On Linux >= 3.19 with Xen, the screen output is broken, and these 
similar messages are showed.
(XEN) [VT-D]DMAR:[DMA Write] Request device [0000:00:02.0] fault addr 
73fbff000, iommu reg = ffff82c000203000
(XEN) [VT-D]DMAR: reason 05 - PTE Write access is not set

> Jan

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