[PATCH v3 11/11] vfio/pci: Provide opt-out for CXL Type-2 extensions

[<mhonap@nvidia.com>]

From: Manish Honap <mhonap@nvidia.com>

Add an opt-out so users can keep vfio-pci's CXL extensions out of the
path for individual devices or for an entire vfio-pci instance.  The
build-time gate is CONFIG_VFIO_PCI_CXL; the runtime gates are:

  - Module parameter vfio_pci.disable_cxl (bool, 0444).  Setting
    disable_cxl=1 at modprobe time makes vfio_pci_probe() set
    vdev->disable_cxl on every device it binds.

  - Variant drivers (mlx5, pds, hisi, nvgrace, xe, etc.) may set
    vdev->disable_cxl=true in their own probe for per-device control
    without needing the module parameter.  The bit lives on
    struct vfio_pci_core_device so it's reachable from any variant.

vfio_pci_cxl_acquire() consults vdev->disable_cxl as the very first
check and returns -ENODEV when set, which makes vfio-pci-core treat
the device as a plain (non-CXL) PCI passthrough — no CAP_CXL, no HDM
or COMP_REGS VFIO regions, no DVSEC clipping shim.

This mirrors the long-standing disable_denylist opt-out shape.

Signed-off-by: Manish Honap <mhonap@nvidia.com>
---
 drivers/vfio/pci/cxl/vfio_cxl_core.c | 9 +++++++++
 drivers/vfio/pci/vfio_pci.c          | 9 +++++++++
 include/linux/vfio_pci_core.h        | 1 +
 3 files changed, 19 insertions(+)

diff --git a/drivers/vfio/pci/cxl/vfio_cxl_core.c b/drivers/vfio/pci/cxl/vfio_cxl_core.c
index 8a00b776d7c7..905f74f4e725 100644
--- a/drivers/vfio/pci/cxl/vfio_cxl_core.c
+++ b/drivers/vfio/pci/cxl/vfio_cxl_core.c
@@ -234,6 +234,15 @@ int vfio_pci_cxl_acquire(struct vfio_pci_core_device *vdev)
 	u16 dvsec;
 	int rc;
 
+	/*
+	 * Honour the per-device opt-out (set by vfio-pci's module
+	 * parameter disable_cxl, or by a variant driver before
+	 * registration).  Returning -ENODEV here makes the caller
+	 * treat this device as plain vfio-pci.
+	 */
+	if (vdev->disable_cxl)
+		return -ENODEV;
+
 	if (!pcie_is_cxl(pdev))
 		return -ENODEV;
 
diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 0c771064c0b8..fd226cb65d8b 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -60,6 +60,12 @@ static bool disable_denylist;
 module_param(disable_denylist, bool, 0444);
 MODULE_PARM_DESC(disable_denylist, "Disable use of device denylist. Disabling the denylist allows binding to devices with known errata that may lead to exploitable stability or security issues when accessed by untrusted users.");
 
+#if IS_ENABLED(CONFIG_VFIO_PCI_CXL)
+static bool disable_cxl;
+module_param(disable_cxl, bool, 0444);
+MODULE_PARM_DESC(disable_cxl, "Disable CXL Type-2 extensions for all devices bound to vfio-pci. Variant drivers may instead set vdev->disable_cxl in their probe for per-device control without needing this parameter.");
+#endif
+
 static bool vfio_pci_dev_in_denylist(struct pci_dev *pdev)
 {
 	switch (pdev->vendor) {
@@ -166,6 +172,9 @@ static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 		return PTR_ERR(vdev);
 
 	dev_set_drvdata(&pdev->dev, vdev);
+#if IS_ENABLED(CONFIG_VFIO_PCI_CXL)
+	vdev->disable_cxl = disable_cxl;
+#endif
 	vdev->pci_ops = &vfio_pci_dev_ops;
 	ret = vfio_pci_core_register_device(vdev);
 	if (ret)
diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
index 541c1911e090..20e9599b3bd7 100644
--- a/include/linux/vfio_pci_core.h
+++ b/include/linux/vfio_pci_core.h
@@ -127,6 +127,7 @@ struct vfio_pci_core_device {
 	bool			needs_pm_restore:1;
 	bool			pm_intx_masked:1;
 	bool			pm_runtime_engaged:1;
+	bool			disable_cxl:1;
 	struct pci_saved_state	*pci_saved_state;
 	struct pci_saved_state	*pm_save;
 	int			ioeventfds_nr;
-- 
2.25.1