Trying to test my gart/iommu vmcore problem on RH

[Eric W. Biederman]

Vivek Goyal <vgoyal at redhat.com> writes:

> Hi Bob,
>
> I am CCing this thread to kexec mailing list. It is good to discuss
> the issue there to get the ideas.
>
> I will summarize the discussion so far.
>
> Bob is running into MCA in second kernel in kdump. Reason seems to be
> that second kernel is trying to access the memory area marked as
> GART aperture (by first kernel). Because GART aperture does not appear
> as "reserved" or something else in /proc/iomem (in case kernel has
> overridden the BIOS settings and has reserved a memory area), second
> kernel thinks it is a valid RAM area and tries to dump it and runs into
> issues.
>
> Few options Bob is considering are.
>
> - Update "e820" memory map to mark GART aperture as reserved, which will
>   be reflected in /proc/iomem also. Kexec-tools will not pass reserved
>   area to second kernel and it will not try to dump this area.
>
>
> - Mark GART aperture as "GART aperture" in /proc/iomem and modify
>   kexec-tools to filter out this memroy from memory map passed to second
>   kernel.


We should definitely reserve the resource, and it should definitely
show up in /proc/iomem.

> - Disable cpu side GART access in first kernel so that even if second
>   kernel tries to access it, it does not run into isseus.

This is an interesting one.  When I looked at this years ago I had the
feeling that if we did this we could actually always use a 2G Aperture
at a fixed address, and require going through the gart for all of lowmem.

But that is a little more than we are talking about.

Eric