[PATCH v4 25/27] vfio/pci: Provide an opt-out for the 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
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 (nvgrace, mlx5, and others) may set vdev->disable_cxl
    in their own probe for per-device control without the module
    parameter. The bit lives on struct vfio_pci_core_device so it is
    reachable from any variant.

vfio_pci_core_init_dev() consults vdev->disable_cxl before it probes for
a CXL device, so a device that opts out is driven as plain vfio-pci:
vfio-cxl is not loaded, init_device() never runs, and the device gets no
VFIO_DEVICE_FLAGS_CXL, no HDM or component-register regions, and no DVSEC
virtualization.

The module parameter is built only when CONFIG_VFIO_CXL is enabled; the
disable_cxl bit itself is unconditional so a variant driver can set it
regardless. This mirrors the long-standing disable_denylist opt-out.

Signed-off-by: Manish Honap <mhonap@nvidia.com>
---
 drivers/vfio/pci/vfio_pci.c      | 9 +++++++++
 drivers/vfio/pci/vfio_pci_core.c | 8 +++++---
 include/linux/vfio_pci_core.h    | 1 +
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 830369ff878d..0ad041fffe48 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_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. A variant driver may instead set vdev->disable_cxl in its own .init callback.");
+#endif
+
 static bool vfio_pci_dev_in_denylist(struct pci_dev *pdev)
 {
 	switch (pdev->vendor) {
@@ -142,6 +148,9 @@ static int vfio_pci_init_dev(struct vfio_device *core_vdev)
 #ifdef CONFIG_VFIO_PCI_VGA
 	vdev->disable_vga = disable_vga;
 #endif
+#if IS_ENABLED(CONFIG_VFIO_CXL)
+	vdev->disable_cxl = disable_cxl;
+#endif
 
 	return vfio_pci_core_init_dev(core_vdev);
 }
diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 0fed8e00bc1d..4b51a0f1e847 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -2423,10 +2423,12 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
 	xa_init(&vdev->ctx);
 
 	/*
-	 * Load vfio-cxl on demand for a CXL device. If it is absent, drive the
-	 * device as plain vfio-pci rather than failing the bind.
+	 * Load vfio-cxl on demand for a CXL device unless the user opted out.
+	 * If it is opted out or absent, drive the device as plain vfio-pci
+	 * rather than failing the bind.
 	 */
-	if (pcie_is_cxl(vdev->pdev) && vfio_pci_is_cxl_type2(vdev->pdev)) {
+	if (!vdev->disable_cxl && pcie_is_cxl(vdev->pdev) &&
+	    vfio_pci_is_cxl_type2(vdev->pdev)) {
 		const struct vfio_cxl_ops *ops;
 
 		request_module("vfio-cxl");
diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
index 18e206a35d8c..8e3723a55c17 100644
--- a/include/linux/vfio_pci_core.h
+++ b/include/linux/vfio_pci_core.h
@@ -158,6 +158,7 @@ struct vfio_pci_core_device {
 	bool			disable_idle_d3:1;
 	bool			nointxmask:1;
 	bool			disable_vga:1;
+	bool			disable_cxl:1;
 	/* Flags modified at runtime - dedicated storage unit */
 	bool			needs_reset;
 	bool			pm_intx_masked;
-- 
2.25.1