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

[Baolu Lu <baolu.lu@linux.intel.com>]

+Jason

On 1/5/23 11:27 PM, Felix Kuehling wrote:
> Am 2023-01-05 um 09:46 schrieb Deucher, Alexander:
>> [AMD Official Use Only - General]
>>
>>> -----Original Message-----
>>> From: Hegde, Vasant <Vasant.Hegde@amd.com>
>>> Sent: Thursday, January 5, 2023 5:46 AM
>>> To: Baolu Lu <baolu.lu@linux.intel.com>; Matt Fagnani
>>> <matt.fagnani@bell.net>; Thorsten Leemhuis <regressions@leemhuis.info>;
>>> Deucher, Alexander <Alexander.Deucher@amd.com>; Joerg Roedel
>>> <jroedel@suse.de>
>>> Cc: iommu@lists.linux.dev; LKML <linux-kernel@vger.kernel.org>;
>>> regressions@lists.linux.dev; Linux PCI <linux-pci@vger.kernel.org>; 
>>> Bjorn
>>> Helgaas <bhelgaas@google.com>
>>> Subject: Re: [regression, bisected, pci/iommu] Bug 216865 - Black screen
>>> when amdgpu started during 6.2-rc1 boot with AMD IOMMU enabled
>>>
>>> Baolu,
>>>
>>>
>>> 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.
>>>>> Thanks for testing. As mentioned earlier I was not expecting this
>>>>> patch to fix the black screen issue. It should fix kernel warnings
>>>>> and IOMMU page fault related call traces. By any chance do you have 
>>>>> the
>>> kernel boot logs?
>>>>> @Baolu,
>>>>>     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?
>>> Honestly I don't know the complete details of how PASID works with 
>>> graphics
>>> card. May be Alex or Joerg can explain it better.
>> + Felix
>>
>> 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.

--
Best regards,
baolu