Re: [RFC v2 07/15] vfio/cxl: expose CXL region to the userspace via a new VFIO device region

Dave Jiang
2025-12-11T16:06:15Z

On 12/9/25 9:50 AM, mhonap@nvidia.com wrote:
> From: Manish Honap <mhonap@nvidia.com>
>
> To directly access the device memory, a CXL region is required. Creating
> a CXL region requires to configure HDM decoders on the path to map the
> access of HPA level by level and evetually hit the DPA in the CXL
> topology.
>
> For the userspace, e.g. QEMU, to access the CXL region, the region is
> required to be exposed via VFIO interfaces.
>
> Introduce a new VFIO device region and region ops to expose the created
> CXL region when initialize the device in the vfio-cxl-core. Introduce a
> new sub region type for the userspace to identify a CXL region.
>
> Co-developed-by: Zhi Wang <zhiw@nvidia.com>
> Signed-off-by: Zhi Wang <zhiw@nvidia.com>
> Signed-off-by: Manish Honap <mhonap@nvidia.com>
> ---
> drivers/vfio/pci/vfio_cxl_core.c | 122 +++++++++++++++++++++++++++++++
> drivers/vfio/pci/vfio_pci_core.c | 3 +-
> include/linux/vfio_pci_core.h | 5 ++
> include/uapi/linux/vfio.h | 4 +
> 4 files changed, 133 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vfio/pci/vfio_cxl_core.c b/drivers/vfio/pci/vfio_cxl_core.c
> index cf53720c0cb7..35d95de47fa8 100644
> --- a/drivers/vfio/pci/vfio_cxl_core.c
> +++ b/drivers/vfio/pci/vfio_cxl_core.c
> @@ -231,6 +231,128 @@ void vfio_cxl_core_destroy_cxl_region(struct vfio_cxl_core_device *cxl)
> }
> EXPORT_SYMBOL_GPL(vfio_cxl_core_destroy_cxl_region);
>
> +static int vfio_cxl_region_mmap(struct vfio_pci_core_device *pci,
> + struct vfio_pci_region *region,
> + struct vm_area_struct *vma)
> +{
> + struct vfio_cxl_region *cxl_region = region->data;
> + u64 req_len, pgoff, req_start, end;
> + int ret;
> +
> + if (!(region->flags & VFIO_REGION_INFO_FLAG_MMAP))
> + return -EINVAL;
> +
> + if (!(region->flags & VFIO_REGION_INFO_FLAG_READ) &&
> + (vma->vm_flags & VM_READ))
> + return -EPERM;
> +
> + if (!(region->flags & VFIO_REGION_INFO_FLAG_WRITE) &&
> + (vma->vm_flags & VM_WRITE))
> + return -EPERM;
> +
> + pgoff = vma->vm_pgoff &
> + ((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
> +
> + if (check_sub_overflow(vma->vm_end, vma->vm_start, &req_len) ||
> + check_add_overflow(PHYS_PFN(cxl_region->addr), pgoff, &req_start) ||
> + check_add_overflow(PFN_PHYS(pgoff), req_len, &end))
> + return -EOVERFLOW;
> +
> + if (end > cxl_region->size)
> + return -EINVAL;
> +
> + if (cxl_region->noncached)
> + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> + vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> +
> + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP |
> + VM_DONTEXPAND | VM_DONTDUMP);
> +
> + ret = remap_pfn_range(vma, vma->vm_start, req_start,
> + req_len, vma->vm_page_prot);
> + if (ret)
> + return ret;
> +
> + vma->vm_pgoff = req_start;
> +
> + return 0;
> +}
> +
> +static ssize_t vfio_cxl_region_rw(struct vfio_pci_core_device *core_dev,
> + char __user *buf, size_t count, loff_t *ppos,
> + bool iswrite)
> +{
> + unsigned int i = VFIO_PCI_OFFSET_TO_INDEX(*ppos) - VFIO_PCI_NUM_REGIONS;
> + struct vfio_cxl_region *cxl_region = core_dev->region[i].data;
> + loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
> +
> + if (!count)
> + return 0;
> +
> + return vfio_pci_core_do_io_rw(core_dev, false,
> + cxl_region->vaddr,
> + (char __user *)buf, pos, count,
> + 0, 0, iswrite);
> +}
> +
> +static void vfio_cxl_region_release(struct vfio_pci_core_device *vdev,
> + struct vfio_pci_region *region)
> +{
> +}
> +
> +static const struct vfio_pci_regops vfio_cxl_regops = {
> + .rw = vfio_cxl_region_rw,
> + .mmap = vfio_cxl_region_mmap,
> + .release = vfio_cxl_region_release,
> +};
> +
> +int vfio_cxl_core_register_cxl_region(struct vfio_cxl_core_device *cxl)
> +{
> + struct vfio_pci_core_device *pci = &cxl->pci_core;
> + struct vfio_cxl *cxl_core = cxl->cxl_core;
> + u32 flags;
> + int ret;
> +
> + if (WARN_ON(!cxl_core->region.region || cxl_core->region.vaddr))
> + return -EEXIST;
> +
> + cxl_core->region.vaddr = ioremap(cxl_core->region.addr, cxl_core->region.size);
> + if (!cxl_core->region.addr)

I think you are wanting to check cxl_core->region.vaddr here right?

Also, what is the ioremap'd region for?

DJ

> + return -EFAULT;
> +
> + flags = VFIO_REGION_INFO_FLAG_READ |
> + VFIO_REGION_INFO_FLAG_WRITE |
> + VFIO_REGION_INFO_FLAG_MMAP;
> +
> + ret = vfio_pci_core_register_dev_region(pci,
> + PCI_VENDOR_ID_CXL |
> + VFIO_REGION_TYPE_PCI_VENDOR_TYPE,
> + VFIO_REGION_SUBTYPE_CXL,
> + &vfio_cxl_regops,
> + cxl_core->region.size, flags,
> + &cxl_core->region);
> + if (ret) {
> + iounmap(cxl_core->region.vaddr);
> + cxl_core->region.vaddr = NULL;
> + return ret;
> + }
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(vfio_cxl_core_register_cxl_region);
> +
> +void vfio_cxl_core_unregister_cxl_region(struct vfio_cxl_core_device *cxl)
> +{
> + struct vfio_cxl *cxl_core = cxl->cxl_core;
> +
> + if (WARN_ON(!cxl_core->region.region || !cxl_core->region.vaddr))
> + return;
> +
> + iounmap(cxl_core->region.vaddr);
> + cxl_core->region.vaddr = NULL;
> +}
> +EXPORT_SYMBOL_GPL(vfio_cxl_core_unregister_cxl_region);
> +
> MODULE_LICENSE("GPL");
> MODULE_AUTHOR(DRIVER_AUTHOR);
> MODULE_DESCRIPTION(DRIVER_DESC);
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 7dcf5439dedc..c0695b5db66d 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1698,12 +1698,13 @@ static vm_fault_t vfio_pci_mmap_page_fault(struct vm_fault *vmf)
> return vfio_pci_mmap_huge_fault(vmf, 0);
> }
>
> -static const struct vm_operations_struct vfio_pci_mmap_ops = {
> +const struct vm_operations_struct vfio_pci_mmap_ops = {
> .fault = vfio_pci_mmap_page_fault,
> #ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> .huge_fault = vfio_pci_mmap_huge_fault,
> #endif
> };
> +EXPORT_SYMBOL_GPL(vfio_pci_mmap_ops);
>
> int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma)
> {
> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> index a343b91d2580..3474835f5d65 100644
> --- a/include/linux/vfio_pci_core.h
> +++ b/include/linux/vfio_pci_core.h
> @@ -102,6 +102,7 @@ struct vfio_cxl_region {
> struct cxl_region *region;
> u64 size;
> u64 addr;
> + void *vaddr;
> bool noncached;
> };
>
> @@ -203,6 +204,8 @@ vfio_pci_core_to_cxl(struct vfio_pci_core_device *pci)
> return container_of(pci, struct vfio_cxl_core_device, pci_core);
> }
>
> +extern const struct vm_operations_struct vfio_pci_mmap_ops;
> +
> int vfio_cxl_core_enable(struct vfio_cxl_core_device *cxl,
> struct vfio_cxl_dev_info *info);
> void vfio_cxl_core_finish_enable(struct vfio_cxl_core_device *cxl);
> @@ -210,5 +213,7 @@ void vfio_cxl_core_disable(struct vfio_cxl_core_device *cxl);
> void vfio_cxl_core_close_device(struct vfio_device *vdev);
> int vfio_cxl_core_create_cxl_region(struct vfio_cxl_core_device *cxl, u64 size);
> void vfio_cxl_core_destroy_cxl_region(struct vfio_cxl_core_device *cxl);
> +int vfio_cxl_core_register_cxl_region(struct vfio_cxl_core_device *cxl);
> +void vfio_cxl_core_unregister_cxl_region(struct vfio_cxl_core_device *cxl);
>
> #endif /* VFIO_PCI_CORE_H */
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 75100bf009ba..95be987d2ed5 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -372,6 +372,10 @@ struct vfio_region_info_cap_type {
> /* sub-types for VFIO_REGION_TYPE_GFX */
> #define VFIO_REGION_SUBTYPE_GFX_EDID (1)
>
> +/* 1e98 vendor PCI sub-types */
> +/* sub-type for VFIO CXL region */
> +#define VFIO_REGION_SUBTYPE_CXL (1)
> +
> /**
> * struct vfio_region_gfx_edid - EDID region layout.
> *