Re: [regression, bisected, pci/iommu] Bug 216865 - Black screen when amdgpu started during 6.2-rc1 boot with AMD IOMMU enabled

[Felix Kuehling]

[+Shimmer, Aaron]

Am 2023-02-15 um 10:39 schrieb Bjorn Helgaas:
> [+cc Christian, Xinhui, amd-gfx]
>
> On Fri, Jan 06, 2023 at 01:48:11PM +0800, Baolu Lu wrote:
>> On 1/5/23 11:27 PM, Felix Kuehling wrote:
>>> Am 2023-01-05 um 09:46 schrieb Deucher, Alexander:
>>>>> -----Original Message-----
>>>>> From: Hegde, Vasant <Vasant.Hegde at amd.com>
>>>>> On 1/5/2023 4:07 PM, Baolu Lu wrote:
>>>>>> On 2023/1/5 18:27, Vasant Hegde wrote:
>>>>>>> On 1/5/2023 6:39 AM, Matt Fagnani wrote:
>>>>>>>> I built 6.2-rc2 with the patch applied. The same black
>>>>>>>> screen problem happened with 6.2-rc2 with the patch. I
>>>>>>>> tried to use early kdump with 6.2-rc2 with the patch
>>>>>>>> twice by panicking the kernel with sysrq+alt+c after the
>>>>>>>> black screen happened. The system rebooted after about
>>>>>>>> 10-20 seconds both times, but no kdump and dmesg files
>>>>>>>> were saved in /var/crash. I'm attaching the lspci -vvv
>>>>>>>> output as requested. ...
>>>>>>> Looking into lspci output, it doesn't list ACS feature
>>>>>>> for Graphics card. So with your fix it didn't enable PASID
>>>>>>> and hence it failed to boot. ...
>>>>>> So do you mind telling why does the PASID need to be enabled
>>>>>> for the graphic device? Or in another word, what does the
>>>>>> graphic driver use the PASID for? ...
>>>> The GPU driver uses the pasid for shared virtual memory between
>>>> the CPU and GPU.  I.e., so that the user apps can use the same
>>>> virtual address space on the GPU and the CPU.  It also uses
>>>> pasid to take advantage of recoverable device page faults using
>>>> PRS. ...
>>> Agreed. This applies to GPU computing on some older AMD APUs that
>>> take advantage of memory coherence and IOMMUv2 address translation
>>> to create a shared virtual address space between the CPU and GPU.
>>> In this case it seems to be a Carrizo APU. It is also true for
>>> Raven APUs. ...
>> Thanks for the explanation.
>>
>> This is actually the problem that commit 201007ef707a was trying to
>> fix.  The PCIe fabric routes Memory Requests based on the TLP
>> address, ignoring any PASID (PCIe r6.0, sec 2.2.10.4), so a TLP with
>> PASID that should go upstream to the IOMMU may instead be routed as
>> a P2P Request if its address falls in a bridge window.
>>
>> In SVA case, the IOMMU shares the address space of a user
>> application.  The user application side has no knowledge about the
>> PCI bridge window.  It is entirely possible that the device is
>> programed with a P2P address and results in a disaster.
> Is this stalled?  We explored the idea of changing the PCI core so
> that for devices that use ATS/PRI, we could enable PASID without
> checking for ACS [1], but IIUC we ultimately concluded that it was
> based on a misunderstanding of how ATS Translation Requests are routed
> and that an AMD driver change would be required [2].
>
> So it seems like we still have this regression, and we're running out
> of time before v6.2.
>
> [1] https://lore.kernel.org/all/20230114073420.759989-1-baolu.lu@linux.intel.com/
> [2] https://lore.kernel.org/all/Y91X9MeCOsa67CC6@nvidia.com/

If I understand this correctly, the HW or the BIOS is doing something 
wrong about reporting ACS. I don't know what the GPU driver can do other 
than add some quirk to stop using AMD IOMMUv2 on this HW/BIOS.

It looks like the problem is triggered when the driver calls 
amd_iommu_init_device. That's when the first WARNs happen, soon followed 
by a kernel oops in report_iommu_fault. The driver doesn't know anything 
is wrong because amd_iommu_init_device seems to return "success". And 
the oops is not in the GPU driver either.

I guess this could also be handled more gracefully in the IOMMU driver 
(i.e. fail gracefully in amd_iommu_init_device and let the caller know 
that something is wrong, don't oops in report_iommu_fault).

Regards,
   Felix