linux-kernel - Re: [PATCH v3 5/9] cxl/pci: Only register RCDs with device 0, function 0 as CXL memory device

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Date:   Thu, 17 Nov 2022 16:56:25 +0100
From:   Robert Richter <rrichter@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v3 5/9] cxl/pci: Only register RCDs with device 0,
 function 0 as CXL memory device

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@....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.

Note: There can be multiple devices on the bus, but those shouldn't
have a CXL mem dev class code and the devices shouldn't being probed
by cxl_pci_probe() which contains the check and later creates a cxlmd
dev.

-Robert