Re: iommu=no-igfx

[buy computer <buycomputer40@xxxxxxxxx>]

On Mon, May 25, 2020 at 5:16 AM Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Wednesday, May 20, 2020 7:11 PM
>
> On 11.05.2020 19:43, buy computer wrote:
> > I've been working on a Windows 10 HVM on a Debian 10 dom0. When I
> was first
> > trying to make the VM, I was getting IOMMU errors. I had a hard time
> > figuring out what to do about this, and finally discovered that putting
> > iommu=no-igfx in the grub stopped the errors.
> >
> > Unfortunately, without the graphics support the VM is understandably
> slow,
> > and can crash. I was also only now pointed to the page
> > <https://xenbits.xen.org/docs/unstable/misc/xen-command-
> line.html#iommu>
> > which says to report any errors that get fixed by using iommu=no-igfx.

what is the platform and linux kernel version in this context?

I'm not sure what you meant by 'platform', so I'll try to cover all the bases.

Kernel: 4.19.0-9-amd64 GNU/Linux

Debian 10.4

Lenovo E490 ThinkPad

Intel Integrated Graphics 620

 

>
> Thanks for the report. For context I'll quote the commit message of
> the commit introducing the option as well as the request to report
> issues fixed with it:
>
> "As we still cannot find a proper fix for this problem, this patch adds
>  iommu=igfx option to control whether Intel graphics IOMMU is enabled.
>  Running Xen with iommu=no-igfx is similar to running Linux with
>  intel_iommu=igfx_off, which disables IOMMU for Intel GPU. This can be
>  used by users to manually workaround the problem before a fix is
>  available for i915 driver."
>
> This was in 2015, referencing Linux >= 3.19. I have no idea whether
> the underlying driver issue(s) has/have been fixed. The addresses
> referenced are variable enough and all within RAM, so I'd conclude
> this is not a "missing RMRR" issue.

Variable enough but not within RAM. From E820:

(XEN)  0000000100000000 - 0000000871800000 (usable)

But the referenced addresses are way higher:

(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault
addr 76c615d000, iommu reg = ffff82c000a0c000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set

>
> Cc-ing the VT-d maintainer for possible insights or thoughts.
>
> Jan

I don't have other thoughts except the weird addresses. It might be
good to add some trace in dom0's i915 driver to see whether those
addresses are intended or not.

Thanks for the insight! I'd love to help with the trace, but I don't know how to do that. If you could point me in the right direction, I'd try to give it a shot.

Thanks
Kevin

Thanks for the insight!