Re: [PATCH v4 25/27] vfio/pci: Provide an opt-out for the CXL Type-2 extensions

[Alex Williamson <alex@shazbot.org>]

On Thu, 13 Aug 2026 15:06:29 +0530
<mhonap@nvidia.com> wrote:

> 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.");

The latter sentence isn't module parameter description material, it's
aimed at users.  Why does this need to be read-only since it's
following the latch-at-init flow?  Thanks,

Alex

> +#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;