Re: [PATCH v3 5/6] PCI/AER: Forward RCH downstream port-detected errors to the CXL.mem dev handler

[Robert Richter]

Dan,

thanks for review, see comments inline.

On 17.04.23 18:01:41, Dan Williams wrote:
> Terry Bowman wrote:
> > From: Robert Richter <rrichter@xxxxxxx>
> > 
> > In Restricted CXL Device (RCD) mode a CXL device is exposed as an
> > RCiEP, but CXL downstream and upstream ports are not enumerated and
> > not visible in the PCIe hierarchy. Protocol and link errors are sent
> > to an RCEC.
> > 
> > Restricted CXL host (RCH) downstream port-detected errors are signaled
> > as internal AER errors, either Uncorrectable Internal Error (UIE) or
> > Corrected Internal Errors (CIE). The error source is the id of the
> > RCEC. A CXL handler must then inspect the error status in various CXL
> > registers residing in the dport's component register space (CXL RAS
> > cap) or the dport's RCRB (AER ext cap). [1]
> > 
> > Errors showing up in the RCEC's error handler must be handled and
> > connected to the CXL subsystem. Implement this by forwarding the error
> > to all CXL devices below the RCEC. Since the entire CXL device is
> > controlled only using PCIe Configuration Space of device 0, Function
> > 0, only pass it there [2]. These devices have the Memory Device class
> > code set (PCI_CLASS_MEMORY_CXL, 502h) and the existing cxl_pci driver
> > can implement the handler. In addition to errors directed to the CXL
> > endpoint device, the handler must also inspect the CXL downstream
> > port's CXL RAS and PCIe AER external capabilities that is connected to
> > the device.
> > 
> > Since CXL downstream port errors are signaled using internal errors,
> > the handler requires those errors to be unmasked. This is subject of a
> > follow-on patch.
> > 
> > The reason for choosing this implementation is that a CXL RCEC device
> > is bound to the AER port driver, but the driver does not allow it to
> > register a custom specific handler to support CXL. Connecting the RCEC
> > hard-wired with a CXL handler does not work, as the CXL subsystem
> > might not be present all the time. The alternative to add an
> > implementation to the portdrv to allow the registration of a custom
> > RCEC error handler isn't worth doing it as CXL would be its only user.
> > Instead, just check for an CXL RCEC and pass it down to the connected
> > CXL device's error handler. With this approach the code can entirely
> > be implemented in the PCIe AER driver and is independent of the CXL
> > subsystem. The CXL driver only provides the handler.
> > 
> > [1] CXL 3.0 spec, 12.2.1.1 RCH Downstream Port-detected Errors
> > [2] CXL 3.0 spec, 8.1.3 PCIe DVSEC for CXL Devices
> > 
> > Co-developed-by: Terry Bowman <terry.bowman@xxxxxxx>
> > Signed-off-by: Robert Richter <rrichter@xxxxxxx>
> > Signed-off-by: Terry Bowman <terry.bowman@xxxxxxx>
> > Cc: "Oliver O'Halloran" <oohall@xxxxxxxxx>
> > Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > Cc: Mahesh J Salgaonkar <mahesh@xxxxxxxxxxxxx>
> > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> > Cc: linux-pci@xxxxxxxxxxxxxxx
> > ---
> >  drivers/pci/pcie/Kconfig |  8 ++++++
> >  drivers/pci/pcie/aer.c   | 61 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 69 insertions(+)
> > 
> > diff --git a/drivers/pci/pcie/Kconfig b/drivers/pci/pcie/Kconfig
> > index 228652a59f27..b0dbd864d3a3 100644
> > --- a/drivers/pci/pcie/Kconfig
> > +++ b/drivers/pci/pcie/Kconfig
> > @@ -49,6 +49,14 @@ config PCIEAER_INJECT
> >  	  gotten from:
> >  	     https://git.kernel.org/cgit/linux/kernel/git/gong.chen/aer-inject.git/
> >  
> > +config PCIEAER_CXL
> > +	bool "PCI Express CXL RAS support"
> > +	default y
> > +	depends on PCIEAER && CXL_PCI
> > +	help
> > +	  This enables CXL error handling for Restricted CXL Hosts
> > +	  (RCHs).
> > +
> >  #
> >  # PCI Express ECRC
> >  #
> > diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
> > index 7a25b62d9e01..171a08fd8ebd 100644
> > --- a/drivers/pci/pcie/aer.c
> > +++ b/drivers/pci/pcie/aer.c
> > @@ -946,6 +946,65 @@ static bool find_source_device(struct pci_dev *parent,
> >  	return true;
> >  }
> >  
> > +#ifdef CONFIG_PCIEAER_CXL
> > +
> > +static bool is_cxl_mem_dev(struct pci_dev *dev)
> > +{
> > +	/*
> > +	 * A CXL device is controlled only using PCIe Configuration
> > +	 * Space of device 0, Function 0.
> > +	 */
> > +	if (dev->devfn != PCI_DEVFN(0, 0))
> > +		return false;
> > +
> > +	/* Right now there is only a CXL.mem driver */
> > +	if ((dev->class >> 8) != PCI_CLASS_MEMORY_CXL)
> > +		return false;
> > +
> > +	return true;
> > +}
> 
> This part feels broken because most the errors of concern here are CXL
> link generic and that can involve CXL.cache and CXL.mem errors on
> devices that are not PCI_CLASS_MEMORY_CXL. This situation feels like it
> wants formal acknowledgement in 'struct pci_dev' that CXL links ride on
> top of PCIe links.

There is already rcec->rcec_ea that holds the RCEC-to-endpoint
association. Determining if the RCiEP is a CXL dev is a small check
which is exactly what is_cxl_mem_dev() is for. I don't see a benefit
in holding the same information in an additional cxl_link structure.

And as you also said below, for RCRB handling a CXL driver is needed
which is why is_cxl_mem_dev() with the class check is used below.

> 
> If it were not for RCRBs then the PCI core could just do:
> 
>     dvsec = pci_find_dvsec_capability(pdev, PCI_DVSEC_VENDOR_ID_CXL,
>                                       CXL_DVSEC_FLEXBUS_PORT);
> 
> ...at bus scan time to identify devices with active CXL links. RCRBs
> unfortunately make it so the link presence can not be detected until a
> CXL driver is loaded to read that DVSEC out of MMIO space.

In a VH topology those errors can be directly handled in a pci driver
for CXL ports, if the portdrv handles that the check could be useful.
But this is not subject of this patch series.

> 
> However, I still think that looks like a CXL aware driver registering a
> 'struct cxl_link' (for lack of a better name) object with a
> corresponding PCI device. That link can indicate whether this is an RCH
> topology and whether it needs to do the RCEC walk, and that registration
> event can flag the RCEC has having CXL link duties to attend to on AER
> events.

For CXL awareness of the AER driver the simple checks from above could
be used, either called directly for the pci_dev (VH mode), or by
walking the RCEC. IMO, a 'struct cxl_link' and a function to register
it are not really needed here.

> 
> I suspect 'struct cxl_link' can also be used if/when we get to
> incoporating CXL Reset into PCI reset handling.
> 
> > +
> > +static bool is_internal_error(struct aer_err_info *info)
> > +{
> > +	if (info->severity == AER_CORRECTABLE)
> > +		return info->status & PCI_ERR_COR_INTERNAL;
> > +
> > +	return info->status & PCI_ERR_UNC_INTN;
> > +}
> > +
> > +static void handle_error_source(struct pci_dev *dev, struct aer_err_info *info);
> > +
> > +static int cxl_handle_error_iter(struct pci_dev *dev, void *data)
> > +{
> > +	struct aer_err_info *e_info = (struct aer_err_info *)data;
> > +
> > +	if (!is_cxl_mem_dev(dev))
> > +		return 0;
> 
> 
> I assume this also needs to reference the RDPAS if present?

That is subject of a follow-on patch.

Here I see, why you may need a struct cxl_link. But that list must not
reside in the pci_dev, instead a CXL aware driver can look up a
self-maintained list of RDPAS mappings (RCEC-to-Downstream Port
assosiations) to decide whether to lookup the dport's AER and RAS
capablilities.

> 
> CXL 3.0 9.17.1.5 RCEC Downstream Port Association Structure (RDPAS)
> 
> > +
> > +	/* pci_dev_put() in handle_error_source() */
> > +	dev = pci_dev_get(dev);
> > +	if (dev)
> > +		handle_error_source(dev, e_info);
> 
> I went looking but missed where does handle_error_source() synchronize
> against driver ->remove()?

Right, the device_lock() is missing in handle_error_source() while
accessing pdrv and calling the handler. Will send a fix.

> 
> > +
> > +	return 0;
> > +}
> > +
> > +static void cxl_handle_error(struct pci_dev *dev, struct aer_err_info *info)
> 
> Naming suggestion...
> 
> Given that the VH topology does not require this scanning and
> assoication step, lets call this cxl_rch_handle_error() to make it clear
> this is only here to undo the awkwardness of CXL 1.1 platforms hiding
> registers from typical PCI scanning. A reference to:
> 
> CXL 3.0 9.11.8 CXL Devices Attached to an RCH
> 
> ...might be useful to a future reader that wonders why the CXL RCH case
> is so complicated from an AER perspective.

Ok.

Thanks,

-Robert