linux-kernel - Re: [PATCH v2 4/5] cxl/pci: Forward RCH downstream port-detected errors to the CXL.mem dev handler

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Message-ID: <20230328172104.GA2897826@bhelgaas>
Date:   Tue, 28 Mar 2023 12:21:04 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Robert Richter <rrichter@....com>
Cc:     Terry Bowman <terry.bowman@....com>, alison.schofield@...el.com,
        vishal.l.verma@...el.com, ira.weiny@...el.com, bwidawsk@...nel.org,
        dan.j.williams@...el.com, dave.jiang@...el.com,
        Jonathan.Cameron@...wei.com, linux-cxl@...r.kernel.org,
        linux-kernel@...r.kernel.org, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Oliver O'Halloran <oohall@...il.com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 4/5] cxl/pci: Forward RCH downstream port-detected
 errors to the CXL.mem dev handler

[+cc linux-pci, more error handling folks; beginning of thread at
https://lore.kernel.org/all/20230323213808.398039-1-terry.bowman@amd.com/]

On Mon, Mar 27, 2023 at 11:51:39PM +0200, Robert Richter wrote:
> On 24.03.23 17:36:56, Bjorn Helgaas wrote:

> > > The CXL device driver is then responsible to
> > > enable error reporting in the RCEC's AER cap
> > 
> > I don't know exactly what you mean by "error reporting in the RCEC's
> > AER cap", but IIUC, for non-Root Port devices, generation of ERR_COR/
> > ERR_NONFATAL/ERR_FATAL messages is controlled by the Device Control
> > register and should already be enabled by pci_aer_init().
> > 
> > Maybe you mean setting AER mask/severity specifically for Internal
> > Errors?  I'm hoping to get as much of AER management as we can in the
> 
> Richt, this is implemented in patch #5 in function
> rcec_enable_aer_ints().

I think we should add a PCI core interface for this so we can enforce
the AER ownership question (all the crud like pcie_aer_is_native()) in
one place.

> > PCI core and out of drivers, so maybe we need a new PCI interface to
> > do that.
> > 
> > In any event, I assume this sort of configuration would be an
> > enumeration-time thing, while *this* patch is a run-time thing, so
> > maybe this information belongs with a different patch?
> 
> Do you mean once a Restricted CXL host (RCH) is detected, the internal
> errors should be enabled in the device mask, all this done during
> device enumeration? But wouldn't interrupts being enabled then before
> the CXL device is ready?

I'm not sure what you mean by "before the CXL device is ready."  What
makes a CXL device ready, and how do we know when it is ready?

pci_aer_init() turns on PCI_EXP_DEVCTL_CERE, PCI_EXP_DEVCTL_FERE, etc
as soon as we enumerate the device, before any driver claims the
device.  I'm wondering whether we can do this PCI_ERR_COR_INTERNAL and
PCI_ERR_UNC_INTN fiddling around the same time?

> > I haven't worked all the way through this, but I thought Sean Kelley's
> > and Qiuxu Zhuo's work was along the same line and might cover this,
> > e.g.,
> > 
> >   a175102b0a82 ("PCI/ERR: Recover from RCEC AER errors")
> >   579086225502 ("PCI/ERR: Recover from RCiEP AER errors")
> >   af113553d961 ("PCI/AER: Add pcie_walk_rcec() to RCEC AER handling")
> > 
> > But I guess maybe it's not quite the same case?
> 
> Actually, we use this code to handle errors that are reported to the
> RCEC and only implement here the CXL specifics. That is, checking if
> the RCEC receives something from a CXL downstream port and forwarding
> that to a CXL handler (this patch). The handler then checks the AER
> err cap in the RCRB of all CXL downstream ports associated to the RCEC
> (not visible in the PCI hierarchy), but discovered through the :00.0
> RCiEP (patch #5).

There are two calls to pcie_walk_rcec():

  1) The existing one in find_source_device()
  2) The one you add in handle_cxl_error()

Does the call in handle_cxl_error() look at devices that the existing
call in find_source_device() does not?  I'm trying to understand why
we need both calls.

> > > +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_cxl_error(struct pci_dev *dev, struct aer_err_info *info)
> > > +{
> > > +	if (pci_pcie_type(dev) == PCI_EXP_TYPE_RC_EC &&
> > > +	    is_internal_error(info))
> > 
> > What's unique about Internal Errors?  I'm trying to figure out why you
> > wouldn't do this for *all* CXL errors.
> 
> Per CXL specification downstream port errors are signaled using
> internal errors. 

Maybe a spec reference here to explain is_internal_error()?  Is the
point of the check to *exclude* non-internal errors?  Or is basically
documentation that there shouldn't ever *be* any non-internal errors?
I guess the latter wouldn't make sense because at this point we don't
know whether this is a CXL hierarchy.

> All other errors would be device specific, we cannot
> handle that in a generic CXL driver.

I'm missing the point here.  We don't have any device-specific error
handling in aer.c; it only connects the generic *reporting* mechanism
(AER log registers and Root Port interrupts) to the drivers that do
the device-specific things via err_handler hooks.  I assume we want a
similar model for CXL.

Bjorn