Re: Kernel 6.7 regression doesn't boot if using AMD eGPU

[Vasant Hegde]

Robin,

On 4/16/2024 4:55 PM, Robin Murphy wrote:
> On 2024-04-16 1:39 am, Jason Gunthorpe wrote:
> > On Mon, Apr 15, 2024 at 10:44:34PM +0100, Robin Murphy wrote:
> > > On 2024-04-15 7:57 pm, Eric Wagner wrote:
> > > > Apologies if I made a mistake in the first bisect, I'm new to kernel
> > > > debugging.
> > > >
> > > > I tested cedc811c76778bdef91d405717acee0de54d8db5 (x86/amd) and
> > > > 3613047280ec42a4e1350fdc1a6dd161ff4008cc (core) directly and both were good.
> > > > Then I ran git bisect again with e8cca466a84a75f8ff2a7a31173c99ee6d1c59d2
> > > > as the bad and 6e6c6d6bc6c96c2477ddfea24a121eb5ee12b7a3 as the good and the
> > > > bisect log is attached. It ended up at the same commit as before.
> > > >
> > > > I've also attached a picture of the boot screen that occurs when it hangs.
> > > > 0000:05:00.0 is the PCIe bus address of the RX 580 eGPU that's causing the
> > > > problem.

../...

> "Failing" iommu_probe_device is merely how we tell ourselves that we're not 
> interested in a device, and consequently tell the rest of the kernel it doesn't 
> have an IOMMU (via device_iommu_mapped() returning false). This is normal and 
> expected for devices which legitimately have no IOMMU in the first place; 
> conversely we don't do a great deal for unexpected failures since those 
> typically represent system-fatal conditions whatever we might try to do. We've 
> never had much of a notion of expected failures when an IOMMU *is* present, but 
> even then, denying any trace of the IOMMU and removing ourselves from the 
> picture is clearly not the ideal way to approach that. We're running off a bus 
> notifier (or even later), so ultimately our return value is meaningless; at that 
> point the device already exists and has been added to its bus, we can't undo that.
>
> However it looks to be even more fun if failure occurs in *deferred* default 
> domain creation via bus_iommu_probe(), since then we give up and dismiss the 
> entire IOMMU. Except the x86 drivers ignore the return from 
> iommu_device_register(), so further hilarity ensues...
>
> I think I've now satisfied myself that a simple fix for the core code is 
> appropriate and will write that up now; one other thing I couldn't quite figure 
> out is whether the AMD driver somehow prevents PASIDs being used while the group 
> is attached to a non-identity (and non-nested) domain - that's probably one for 
> Vasant to confirm.

AMD driver supports PASID with below domain type :
  - Identity domain
  - DMA translation mode (DMA and DMA_FQ) with AMD v2 page table 
(amd_iommu=pgtbl_v2).


Currently amd_iommu_def_domain_type() tries to put PASID capable devices in 
identity domain mode. This is something to fix. Its in my TODO list. I will try 
to get into it soon.

Hope this clarifies.

-Vasant