> -----Original Message-----
> From: Alex Williamson <[email protected]>
> Sent: Saturday, August 29, 2026 4:27 AM
> To: Manish Honap <[email protected]>
> Cc: [email protected]; Ankit Agrawal <[email protected]>; [email protected];
> [email protected]; [email protected]; Srirangan Madhavan
> <[email protected]>; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; Yishai Hadas
> <[email protected]>; Shameer Kolothum Thodi
> <[email protected]>; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected]; Neo Jia
> <[email protected]>; Krishnakant Jaju <[email protected]>; Vikram Sethi
> <[email protected]>; Zhi Wang <[email protected]>; linux-
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; linux-
> [email protected]; [email protected]; [email protected]
> Subject: Re: [PATCH v4 25/27] vfio/pci: Provide an opt-out for the CXL Type-2
> extensions
>
> External email: Use caution opening links or attachments
>
>
> On Thu, 13 Aug 2026 15:06:29 +0530
> <[email protected]> wrote:
>
> > From: Manish Honap <[email protected]>
> >
> > 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 <[email protected]>
> > ---
> > 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
Okay, I will move the user-facing sentence out of the module-parameter
description
and use the normal parameter mode rather than read-only.
Thanks,
Manish
>
> > +#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;
