Re: [PATCH v3 1/1] PCI: Add translated request only flag for pci_enable_pasid()

[Vasant Hegde]

Bjorn,


On 2/1/2023 5:44 AM, Bjorn Helgaas wrote:
> On Tue, Jan 31, 2023 at 08:56:13PM +0800, Baolu Lu wrote:
>> On 2023/1/31 2:38, Bjorn Helgaas wrote:
>>>> PCI: Add translated request only flag for pci_enable_pasid()
>>>>
>>>> The PCIe fabric routes Memory Requests based on the TLP address, ignoring
>>>> the PASID. In order to ensure system integrity, commit 201007ef707a ("PCI:
>>>> Enable PASID only when ACS RR & UF enabled on upstream path") requires
>>>> some ACS features being supported on device's upstream path when enabling
>>>> PCI/PASID.
> 
> Looking up 201007ef707a to see what ensuring system integrity means,
> it prevents Memory Requests with PASID, which should always be routed
> to the RC, from being mistakenly routed as peer-to-peer requests.
> 
>>>> However, above change causes the Linux kernel boots to black screen on a
>>>> system with below graphic device:
>>>
>>> We need a PCIe concept-level description of the issue first, i.e., in
>>> terms of DMA, PASID, ACS, etc.  Then we can mention the AMD GPU issue
>>> as an instance.
>>
>> How about below description?
> 
> Thanks, this is exactly the sort of thing I'm looking for.  But my
> understanding of ATS/PRI/PASID is weak, so I'm still working through
> this.  Tell me when I say something wrong below...
> 
>> PCIe endpoints can use ATS to request DMA remapping hardware to
>> translate an IOVA to its mapped physical address. If the translation is
>> missing or the permissions are insufficient, the PRI is used to trigger
>> an I/O page fault. The IOMMU driver will fill the mapping with desired
>> permissions and return the translated address to the device.
> 
> In PCIe spec language, I think you're saying that a PCIe Function may
> contain an ATC.  If the ATC Capability Enable bit is set, the Function
> can issue Translation Requests.
> 
> The TA (aka IOMMU) will respond with a Translation Completion.  If the
> Completion is a CplD, it contains the translated address and the
> Function can store the entry in its ATC.  I assume the I/O page fault
> case corresponds to a Cpl (with no data) meaning that the TA could not
> translate the address.
> 
> If the TA doesn't have a mapping with the desired permissions, and the
> Function's Page Request Capability Enable bit is set, it may issue a
> Page Request Message.  It's up to the TA/IOMMU to make this message
> visible to the OS, which can make the page resident, create an IOMMU
> mapping, and enable a PRG Response Message.  After the Function
> receives the PRG Response Message, it would issue another Translation
> Request.
> 
>> The translated address is specified by the IOMMU driver. The IOMMU
>> driver ensures that the address is a DMA buffer address instead of any
>> P2P address in the PCI fabric. Therefore, any translated memory request
>> will eventually be routed to IOMMU regardless of whether there is ACS
>> control in the up-streaming path.
> 
> A Memory Request with an address that is not a P2P address, i.e., it
> is not contained in any bridge aperture, will *always* be routed
> toward the RC, won't it?  Isn't that the case regardless of whether
> the address is translated or untranslated, and even regardless of ACS?
> 
> IIUC, ACS basically causes peer-to-peer requests to be routed upstream
> instead of directly to the peer.
> 
> OK, reading this again, I realize that I just restated exactly what
> you had already written, sorry about that.
> 
>> AMD GPU is one of those devices.
> 
> I guess you mean the AMD GPU has ATS, PRI, and PASID Capabilities?
> And furthermore, that the GPU *always* uses Translated addresses with
> PASID?
> 
> So I guess what's going on here is that if:
> 
>   - A device only uses PASID with Translated addresses, and 
>   - those Translated addresses are never P2P addresses, then
>   - those transactions will always be routed to the RC.  
> 
> And this applies even if there is no ACS or ACS doesn't support
> PCI_ACS_RR and PCI_ACS_UF.
> 
> The black screen happens because ... ?
> 
> What can we include in the commit log to help people find this fix?  I
> see these in the bugzilla:
> 
>   WARNING: CPU: 0 PID: 477 at drivers/pci/ats.c:251 pci_disable_pri+0x75/0x80
>   WARNING: CPU: 0 PID: 477 at drivers/pci/ats.c:419 pci_disable_pasid+0x45/0x50
> 
> (These look like defects in pdev_pri_ats_enable(), so really just
> distractions)

Right. We have fixed error handling path in this function. Joerg has queued the fix.

> 
>   kfd kfd: amdgpu: Failed to resume IOMMU for device 1002:9874
>   kfd kfd: amdgpu: device 1002:9874 NOT added due to errors
>   BUG: kernel NULL pointer dereference, address: 0000000000000058
>   RIP: 0010:report_iommu_fault+0x11/0x90
> 
> I couldn't figure out the NULL pointer dereference.  I expected it to
> be from a BUG() or similar in report_iommu_fault(), but I don't see
> that.

Its coming from below path :
  - During system boot IOMMU allocates default domain
  - AMD IOMMU v2 module (iommu_v2) created another domain and tried to attach
devices to new domain.
  - In device attachment path (amd_iommu_attach_device()) it first detaches
device from current domain and tries to attach device to new domain. Here device
attachment failed as PASID enable check failed.
  - We didn't recover from above failure (I have proposed fix for this [1]).
  - So device to domain attachment is not in consistent state.
  - Device tried to do DMA and hit IO fault. Above NULL pointer derefence is
coming from that path as dev to domain setup is not proper.

[1]
https://lore.kernel.org/linux-iommu/20230113135956.5788-1-vasant.hegde@xxxxxxx/T/#t

-Vasant