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Date:   Fri, 18 Nov 2022 12:30:26 -0800
From:   Dan Williams <dan.j.williams@intel.com>
To:     Robert Richter <rrichter@amd.com>,
        Dan Williams <dan.j.williams@intel.com>
CC:     Alison Schofield <alison.schofield@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Ben Widawsky <bwidawsk@kernel.org>,
        <linux-cxl@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>,
        "Jonathan Cameron" <Jonathan.Cameron@huawei.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Dave Jiang <dave.jiang@intel.com>
Subject: Re: [PATCH v3 5/9] cxl/pci: Only register RCDs with device 0,
 function 0 as CXL memory device
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Robert Richter wrote:
> On 18.11.22 08:55:13, Dan Williams wrote:
> > Robert Richter wrote:
> > > On 17.11.22 09:27:23, Dan Williams wrote:
> > > > Robert Richter wrote:
> > > > > On 16.11.22 11:24:48, Dan Williams wrote:
> > > > > > Robert Richter wrote:
> > > > > > > The Device 0, Function 0 DVSEC controls the CXL functionality of the
> > > > > > > entire device. Add a check to prevent registration of any other PCI
> > > > > > > device on the bus as a CXL memory device.
> > > > > > 
> > > > > > Can you reference the specification wording that indicates that the OS
> > > > > > needs to actively avoid these situations, or otherwise point to the real
> > > > > > world scenario where this filtering is needed?
> > > > > 
> > > > > CXL 3.0
> > > > > 
> > > > > 8.1.3 PCIe DVSEC for CXL Device
> > > > > 
> > > > > """
> > > > > An RCD creates a new PCIe enumeration hierarchy. As such, it spawns a new Root Bus
> > > > > and can expose one or more PCIe device numbers and function numbers at this bus
> > > > > number. These are exposed as Root Complex Integrated Endpoints (RCiEP). The PCIe
> > > > > Configuration Space of Device 0, Function 0 shall include the CXL PCIe DVSEC as shown
> > > > > in Figure 8-1.
> > > > > """
> > > > > 
> > > > > """
> > > > > In either case, the capability, status, and control fields in Device 0, Function 0 DVSEC
> > > > > control the CXL functionality of the entire device.
> > > > > """
> > > > > 
> > > > > There are some other occurrences. I think this is even true for VH
> > > > > mode, as multiple CXL devices on the bus are exposed through multiple
> > > > > DSPs or Root Ports.
> > > > > 
> > > > > Anyway, I limited this to an RCD only, esp. because its counterpart
> > > > > would be missing and thus port mapping would fail otherwise. See
> > > > > restricted_host_enumerate_dport() of this series.
> > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > Signed-off-by: Robert Richter <rrichter@amd.com>
> > > > > > > ---
> > > > > > >  drivers/cxl/pci.c | 25 +++++++++++++++++++++++--
> > > > > > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > > > > > > index faeb5d9d7a7a..cc4f206f24b3 100644
> > > > > > > --- a/drivers/cxl/pci.c
> > > > > > > +++ b/drivers/cxl/pci.c
> > > > > > > @@ -428,11 +428,26 @@ static void devm_cxl_pci_create_doe(struct cxl_dev_state *cxlds)
> > > > > > >  	}
> > > > > > >  }
> > > > > > >  
> > > > > > > +static int check_restricted_device(struct pci_dev *pdev, u16 pcie_dvsec)
> > > > > > > +{
> > > > > > > +	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_RC_END)
> > > > > > > +		return 0;		/* no RCD */
> > > > > > > +
> > > > > > > +	if (pdev->devfn == PCI_DEVFN(0, 0) && pcie_dvsec)
> > > > > > > +		return 0;		/* ok */
> > > > > > > +
> > > > > > > +	dev_warn(&pdev->dev, "Skipping RCD: devfn=0x%02x dvsec=%u\n",
> > > > > > 
> > > > > > s/0x%02x/%#02x/
> > > > > > 
> > > > > > > +		pdev->devfn, pcie_dvsec);
> > > > > 
> > > > > Ok.
> > > > > 
> > > > > > This looks like a dev_dbg() to me. Otherwise a warning will always fire
> > > > > > on a benign condition.
> > > > > 
> > > > > I have chosen dev_warn() here as this is a non-compliant unexpected
> > > > > behavior of the device. There are no (legal) cases this may happen. I
> > > > > suppose you are worried about spamming the console here, but that
> > > > > error should be reported somewhere and thus being visible.
> > > > 
> > > > There are so many spec illegal values and conditions that the driver
> > > > could checki, but does not. The reason I am poking here is why does the
> > > > driver need to be explicit about *this* illegal condition versus all the
> > > > other potential conditions? What is the practical end user impact if
> > > > Linux does not include this change? For example, if it is just one
> > > > vendor that made this mistake that can be an explicit quirk.
> > > > 
> > > > A dev_warn() is not necessary for simple quirks.
> > > 
> > > This is not simply a cross check, the driver prevents enablement of
> > > CXL mem devs other than PCI_DEVFN(0, 0). It shouldn't silently drop
> > > out then. It's a device malfunction which should appropriate reported
> > > and not only visible if dbg is enabled.
> > > 
> > > As written above, the check is necessary as the counterpart is missing
> > 
> > It is only necessary if this condition happens in practice, not a
> > theoretically. So I am asking, are you seeing this with an actual device
> > that someone will use in production? If so, that's what pci quirks are
> > for to keep those workarounds organized in a common location.
> 
> I can make it a dev_dbg() message. But I do not understand the ratio
> behind this. This is not a quirk nor a workaround or a fix for
> something. The likely paths are the conditions checked that return 0.
> Only if the unlikely case happens where a CXL mem dev is not a dev 0,
> func 0, a warning is shown to inform the user that this dev is not
> enabled. So yes, this might be theoretical similar to that a driver
> cannot allocate memory. But why not print this as a warning message
> then?
> 
> Anyway, let's make it a dev_dbg().

Sorry for the thrash, lets set aside the the dev_dbg() vs dev_warn()
issue. It is minor compared to *why* this patch needs to be applied. I
would expect all production devices to be spec compliant and not
advertise the CXL memory device class code on anything but function0.

So either, there is a real threat that someone will build such a mistake
and Linux needs to take this action to protect itself, or no one will
ever build such a device and this patch is not needed.

Basically I read the changelog and it answered the "What?" question, but
it did not answer the "Why?" question.