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

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
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
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


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