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

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Date:   Sat, 7 Jan 2023 10:44:46 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Vasant Hegde <vasant.hegde@....com>
Cc:     Matt Fagnani <matt.fagnani@...l.net>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        LKML <linux-kernel@...r.kernel.org>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [regression, bisected, pci/iommu] Bug 216865 - Black screen when amdgpu started during 6.2-rc1 boot with AMD IOMMU enabled

On 1/6/2023 10:14 PM, Jason Gunthorpe wrote:
> On Thu, Jan 05, 2023 at 03:57:28PM +0530, Vasant Hegde wrote:
>> Matt,
>>
>> 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.
> 
> The ACS checks being done are feature of the path not the end point or
> root port.
> 
> If we are expecting ACS on the end port then it is just a bug in how
> the test was written.. The test should be a NOP because there are no
> switches in this topology.
> 
> Looking at it, this seems to just be because pci_enable_pasid is
> calling pci_acs_path_enabled wrong, the only other user is here:
> 
> 	for (bus = pdev->bus; !pci_is_root_bus(bus); bus = bus->parent) {
> 		if (!bus->self)
> 			continue;
> 
> 		if (pci_acs_path_enabled(bus->self, NULL, REQ_ACS_FLAGS))
> 			break;
> 
> 		pdev = bus->self;
> 
> 		group = iommu_group_get(&pdev->dev);
> 		if (group)
> 			return group;
> 	}
> 
> And notice it is calling it on pdev->bus not on pdev itself which
> naturally excludes the end point from the ACS validation.
> 
> So try something like:
> 
> 	if (!pci_acs_path_enabled(pdev->bus->self, NULL, PCI_ACS_RR | PCI_ACS_UF))
> 
> (and probably need to check for null ?)

Yeah! This really is a misuse of pci_acs_path_enabled().

But if @pdev is an endpoint of a multiple function device, perhaps we
still need to check acs on it?

--
Best regards,
baolu