Re: iommu=no-igfx

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

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.

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

Jan