[gentoo-user] Re: Issues with AMD_IOMMU

[Ian Zimmerman]

On 2017-05-14 20:07, Corbin Bird wrote:

> Background / System Info : 2 systems.
> Chipset 990FX, FX-9590 ( -march=bdver2 / Fam15h )
> Chipset 790FX, PhenomII 980 ( -march=amdfam10 / Fam10h )
> Gentoo x86_64, multilib, Kernel 4.9.x
> IOMMU enabled in UEFI, BIOS.

> General Info : ( if you don't know already ... )
>
> The 990FX / 790FX Chipset doesn't have a GART / AGP Aperature or IOMMU
> in it.  The CPU contains the original K8 IOMMU ( v1 ) engineered /
> converted from a GART.
>
> Standard output in '/var/log/dmesg'.
>
> > [    0.000000] AGP: No AGP bridge found
> > [    0.926911] PCI-DMA: Disabling AGP.
> > [    0.927142] PCI-DMA: aperture base @ b4000000 size 65536 KB
> > [    0.927263] PCI-DMA: using GART IOMMU.
> > [    0.927376] PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture

I know we have been over this in an earlier thread "Another x11
problem", but I am still very confused.

What BIOS do you have, on the computer that is BIOS based?  I assume you
have one BIOS based and one EFI based, although you don't really say
that.  I am asking because the setup screen of my BIOS (Award modular)
doesn't mention any of these things at all.

Another source of my confusion is that my computer appears to work _now_
without setting any additional kernel options.  The problem I had (X11
turning the screen all black and never coming back) seems to have fixed
itself with an upgrade of either kernel, X11, or radeon firmware, I
don't remember which (and I probably never knew for sure which).

There are some 2 or 3 BIOS upgrades for my motherboard (GA-880GM-D2H)
but they are all ancient anyway, from 2011 - the same year I built the
computer, and the description Gigabyte gives sounds irrelevant to this
stuff, so I never changed the BIOS.

I have looked at the kernel docs you recommend, but they leave a lot to
be desired in the clarity department.  For example, the general
kernel-parameters doc lists a iommu=pt option, without explanation but
tagged with [X86][IA-64].  But the x86_64/boot-options doc doesn't
mention this value at all.

I guess my high-level confusion is that I don't understand the
relationships between all these bits of hardware, and I also don't
understand why the automatic setup that Linux does wouldn't result in a
normally working system.  So there's no aperture, so steal 64M of
addresses from RAM.  Big deal, no?  What's the problem?

Finally, in your last article in that earlier thread you said that I
should turn on multiple APICs in the kernel.  But I can find no kernel
option for that.  What's the exact CONFIG_* symbol to tweak?

Thanks.

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