Re: Serious AMD-Vi(?) issue

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Thu, Apr 18, 2024 at 09:33:31PM -0700, Elliott Mitchell wrote:
> 
> I suspect this is a case of there is some step which is missing from
> Xen's IOMMU handling.  Perhaps something which Linux does during an early
> DMA setup stage, but the current Xen implementation does lazily?
> Alternatively some flag setting or missing step?
> 
> I should be able to do another test approach in a few weeks, but I would
> love if something could be found sooner.

Turned out to be disturbingly easy to get the first entry when it
happened.  Didn't even need `dbench`, it simply showed once the OS was
fully loaded.  I did get some additional data points.

Appears this requires an AMD IOMMUv2.  A test system with known
functioning AMD IOMMUv1 didn't display the issue at all.

(XEN) AMD-Vi: IO_PAGE_FAULT: DDDD:bb:dd.f d0 addr fffffffdf8000000 flags 0x8 I
(XEN) DDDD:bb:dd.f root @ 83b5f5 (3 levels) dfn=fffffffdf8000
(XEN)   L3[1f7] = 0 np

I find it surprising this required "iommu=debug" to get this level of
detail.  This amount of output seems more appropriate for "verbose".

I strongly prefer to provide snippets.  There is a fair bit of output,
I'm unsure which portion is most pertinent.


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