Date: Wed, 17 Apr 2024 11:36:26 +0100
From: Robin Murphy <robin.murphy@....com>
To: Vasant Hegde <vashegde@....com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Eric Wagner <ewagner12@...il.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>,
 Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: Kernel 6.7 regression doesn't boot if using AMD eGPU

On 2024-04-16 1:44 pm, Vasant Hegde wrote:
> 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.

Ooh, I see you now have GIoV to allow that similarly to how SMMUv3 does 
it - that wasn't in the older version of the spec that I've previously 
been referring to :)

Can you confirm there's no hardware actually been made to the older 
spec, supporting v2 and PASIDs but *not* having GIoV? Otherwise, I think 
you'll still have the problem that if you use the GPA-SPA translation in 
the DTE to implement IOMMU_DOMAIN_DMA for the RID, it makes all the 
PASID GVA-GPA mappings useless for host SVA.

Cheers,
Robin.

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