Re: [RFC PATCH 0/9] Add RAS support for CXL root ports, CXL downstream switch ports, and CXL upstream switch ports

[fan <nifan.cxl@gmail.com>]

On Mon, Jun 17, 2024 at 03:04:02PM -0500, Terry Bowman wrote:
> This patchset provides RAS logging for CXL root ports, CXL downstream
> switch ports, and CXL upstream switch ports. This includes changes to
> use a portdrv notifier chain to communicate CXL AER/RAS errors to a
> cxl_pci callback.
> 
> The first 3 patches prepare for and add an atomic notifier chain to the
> portdrv driver. The portdrv's notifier chain reports the port device's
> AER internal errors to the registered callback(s). The preparation changes
> include a portdrv update to call the uncorrectable handler for PCIe root
> ports and PCIe downstream switch ports. Also, the AER correctable error
> (CE) status is made available to the AER CE handler.
> 
> The next 4 patches are in preparation for adding an atomic notification
> callback in the cxl_pci driver. This is for receiving AER internal error
> events from the portdrv notifier chain. Preparation includes adding RAS
> register block mapping, adding trace functions for logging, and
> refactoring cxl_pci RAS functions for reuse.
> 
> The final 2 patches enable the AER internal error interrupts.
> 
> Testing RAS CE/UCE:
>   QEMU was used for testing CXL root port, CXL downstream switch port, and
>   CXL upstream switch port. The aer-inject tool was used to inject AER and
>   a test patch was used to set the AER CIE/UIE and RAS CE/UCE status during
>   testing. Testing passed with no issues.

Hi Terry,

Could you share a little more about the qemu test setup?
From what I see, it seems currently qemu can only inject error to
type3 devices, is that true? Or how to do that for port devices?
Do we need a hack there?

Also, is the aer-inject tool you mentioned the one currently in the kernel
or something else?
https://elixir.bootlin.com/linux/v6.10-rc6/source/drivers/pci/pcie/aer_inject.c

Thanks,
Fan


>  
>   An AMD platform with the AMD RAS error injection tool was used for
>   testing CXL root port injection. Testing passed with no issues.
> 
>   TODO - regression test CXL1.1 RCH handling.
> 
> Solutions Considered (1-4):
>   Below are solutions that were considered. Solution #4 is
>   implemented in this patchset. 
> 
>   1.) Reassigning portdrv error handler for CXL port devices
>   
>   This solution was based on reassigning the portdrv's CE/UCE err_handler
>   to be CXL cxl_pci driver functions.
>   
>   I started with this solution and once the flow was working I realized
>   the endpoint removal would have to be addressed as well. While this
>   could be resolved it does highlight the odd coupling and dependency
>   between the CXL port devices error handling with cxl_pci endpoint's
>   handlers. Also, the err_handler re-assignment at runtime required
>   ignoring the 'const' definition. I don't believe this should be
>   considered as a possible solution.
>   
>   2.) Update the AER driver to call cxl_pci driver's error handler before
>   calling pci_aer_handle_error()
> 
>   This is similar to the existing RCH port error approach in aer.c.
>   In this solution the AER driver searches for a downstream CXL endpoint
>   to 'handle' detected CXL port protocol errors.
> 
>   This is a good solution to consider if the one presented in this patchset
>   is not acceptable. I was initially reluctant to this approach because it
>   adds more CXL coupling to the AER driver. But, I think this solution
>   would technically work. I believe Ming was working towards this
>   solution.
> 
>   3.) Refactor portdrv
>   The portdrv refactoring solution is to change the portdrv service drivers
>   into PCIe auxiliary drivers. With this change the facility drivers can be
>   associated with a PCIe driver instead fixed bound to the portdrv driver.
> 
>   In this case the CXL port functionality would be added either as a CXL
>   auxiliary driver or as a CXL specific port driver
>   (PCI_CLASS_BRIDGE_PCI_NORMAL).
> 
>   This solution has challenges in the interrupt allocation by separate
>   auxiliary drivers and in binding of a specific driver. Binding is
>   currently based on PCIe class and would require extending the binding
>   logic to support multiple drivers for the same class.
> 
>   Jonathan Cameron is working towards this solution by initially solving
>   for the PMU service driver.[1] It is using the auxiliary bus to associate
>   what were service drivers with the portdrv driver. Using a CXL auxiliary
>   for handling CXL port RAS errors would result in RAS logic called from
>   the cxl_pci and CXL auxiliary drivers. This may need a library driver.
> 
>   4.) Using a portdrv notifier chain/callback for CIE/UIE
>   (Implemented in this patchset)
> 
>   This solution uses a portdrv atomic chain notifier and a cxl_pci
>   callback to handle and log CXL port RAS errors.
>   
>   I chose this after trying solution#1 above. I see a couple advantages to
>   this solution are:
>   - Is general port implementation for CIE/UIE specific handling mentioned
>   in the PCIe spec.[2]
>   - Notifier is used in RAS MCE driver as an existing example.
>   - Does not introduce further CXL dependencies into the AER driver.
>   - The notifier chain provides registration/unregistration and
>   synchronization.
> 
>   A disadvantage of this approach is coupling still exists between the CXL
>   port's driver (portdrv) and the cxl_pci driver. The CXL port device's RAS
>   is handled by a notifier callback in the cxl_pci endpoint driver.
> 
>   Most of the patches in this patchset could be reused to work with
>   solution#3 or solution#2. The atomic notifier could be dropped and
>   instead use an auxiliary device or AER driver awareness. The other
>   changes in this patchset could possibly be reused.
> 
>   [1] Kernel.org -
>   https://lore.kernel.org/all/f4b23710-059a-51b7-9d27-b62e8b358b54@linux.intel.com
>   [2] PCI6.0 - 6.2.10 Internal errors
> 
>  drivers/cxl/core/core.h    |   4 +
>  drivers/cxl/core/pci.c     | 153 ++++++++++++++++++++++++++++++++-----
>  drivers/cxl/core/port.c    |   6 +-
>  drivers/cxl/core/trace.h   |  34 +++++++++
>  drivers/cxl/cxl.h          |  10 +++
>  drivers/cxl/cxlpci.h       |   2 +
>  drivers/cxl/mem.c          |  32 +++++++-
>  drivers/cxl/pci.c          |  19 ++++-
>  drivers/pci/pcie/aer.c     |  10 ++-
>  drivers/pci/pcie/err.c     |  20 +++++
>  drivers/pci/pcie/portdrv.c |  32 ++++++++
>  drivers/pci/pcie/portdrv.h |   2 +
>  include/linux/aer.h        |   6 ++
>  13 files changed, 303 insertions(+), 27 deletions(-)
> 
> 
> base-commit: ca3d4767c8054447ac2a58356080e299a59e05b8
> -- 
> 2.34.1
>