drm/xe: Support for mmap-ing mmio regions

Allow the driver to expose hardware register spaces to userspace
through GEM objects with fake mmap offsets. This can be useful
for userspace-firmware communication, debugging, etc.

v2: Minor doc fix (CI)
v3: Enforce MAP_SHARED (Tejas)
Add fault handler with dummy page (Tejas, Matt Auld)
Store physical address instead of xe_mmio in the GEM object (MattB)
v4: Separate xe_mmio_gem from xe_mmio and make it private (MattB)

Signed-off-by: Ilia Levi <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Reviewed-by: Tejas Upadhyay <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lucas De Marchi <[e-mail]>

drivers/gpu/drm/xe/Makefile

···
 	xe_lrc.o \
 	xe_migrate.o \
 	xe_mmio.o \
+	xe_mmio_gem.o \
 	xe_mocs.o \
 	xe_module.o \
 	xe_nvm.o \

drivers/gpu/drm/xe/xe_mmio_gem.c

···
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2025 Intel Corporation
+ */
+
+#include "xe_mmio_gem.h"
+
+#include <drm/drm_drv.h>
+#include <drm/drm_gem.h>
+#include <drm/drm_managed.h>
+
+#include "xe_device_types.h"
+
+/**
+ * DOC: Exposing MMIO regions to userspace
+ *
+ * In certain cases, the driver may allow userspace to mmap a portion of the hardware registers.
+ *
+ * This can be done as follows:
+ * 1. Call xe_mmio_gem_create() to create a GEM object with an mmap-able fake offset.
+ * 2. Use xe_mmio_gem_mmap_offset() on the created GEM object to retrieve the fake offset.
+ * 3. Provide the fake offset to userspace.
+ * 4. Userspace can call mmap with the fake offset. The length provided to mmap
+ *    must match the size of the GEM object.
+ * 5. When the region is no longer needed, call xe_mmio_gem_destroy() to release the GEM object.
+ *
+ * NOTE: The exposed MMIO region must be page-aligned with regards to its BAR offset and size.
+ *
+ * WARNING: Exposing MMIO regions to userspace can have security and stability implications.
+ * Make sure not to expose any sensitive registers.
+ */
+
+static void xe_mmio_gem_free(struct drm_gem_object *);
+static int xe_mmio_gem_mmap(struct drm_gem_object *, struct vm_area_struct *);
+static vm_fault_t xe_mmio_gem_vm_fault(struct vm_fault *);
+
+struct xe_mmio_gem {
+	struct drm_gem_object base;
+	phys_addr_t phys_addr;
+};
+
+static const struct vm_operations_struct vm_ops = {
+	.open = drm_gem_vm_open,
+	.close = drm_gem_vm_close,
+	.fault = xe_mmio_gem_vm_fault,
+};
+
+static const struct drm_gem_object_funcs xe_mmio_gem_funcs = {
+	.free = xe_mmio_gem_free,
+	.mmap = xe_mmio_gem_mmap,
+	.vm_ops = &vm_ops,
+};
+
+static inline struct xe_mmio_gem *to_xe_mmio_gem(struct drm_gem_object *obj)
+{
+	return container_of(obj, struct xe_mmio_gem, base);
+}
+
+/**
+ * xe_mmio_gem_create - Expose an MMIO region to userspace
+ * @xe: The xe device
+ * @file: DRM file descriptor
+ * @phys_addr: Start of the exposed MMIO region
+ * @size: The size of the exposed MMIO region
+ *
+ * This function creates a GEM object that exposes an MMIO region with an mmap-able
+ * fake offset.
+ *
+ * See: "Exposing MMIO regions to userspace"
+ */
+struct xe_mmio_gem *xe_mmio_gem_create(struct xe_device *xe, struct drm_file *file,
+				       phys_addr_t phys_addr, size_t size)
+{
+	struct xe_mmio_gem *obj;
+	struct drm_gem_object *base;
+	int err;
+
+	if ((phys_addr % PAGE_SIZE != 0) || (size % PAGE_SIZE != 0))
+		return ERR_PTR(-EINVAL);
+
+	obj = kzalloc(sizeof(*obj), GFP_KERNEL);
+	if (!obj)
+		return ERR_PTR(-ENOMEM);
+
+	base = &obj->base;
+	base->funcs = &xe_mmio_gem_funcs;
+	obj->phys_addr = phys_addr;
+
+	drm_gem_private_object_init(&xe->drm, base, size);
+
+	err = drm_gem_create_mmap_offset(base);
+	if (err)
+		goto free_gem;
+
+	err = drm_vma_node_allow(&base->vma_node, file);
+	if (err)
+		goto free_gem;
+
+	return obj;
+
+free_gem:
+	xe_mmio_gem_free(base);
+	return ERR_PTR(err);
+}
+
+/**
+ * xe_mmio_gem_mmap_offset - Return the mmap-able fake offset
+ * @gem: the GEM object created with xe_mmio_gem_create()
+ *
+ * This function returns the mmap-able fake offset allocated during
+ * xe_mmio_gem_create().
+ *
+ * See: "Exposing MMIO regions to userspace"
+ */
+u64 xe_mmio_gem_mmap_offset(struct xe_mmio_gem *gem)
+{
+	return drm_vma_node_offset_addr(&gem->base.vma_node);
+}
+
+static void xe_mmio_gem_free(struct drm_gem_object *base)
+{
+	struct xe_mmio_gem *obj = to_xe_mmio_gem(base);
+
+	drm_gem_object_release(base);
+	kfree(obj);
+}
+
+/**
+ * xe_mmio_gem_destroy - Destroy the GEM object that exposes an MMIO region
+ * @gem: the GEM object to destroy
+ *
+ * This function releases resources associated with the GEM object created by
+ * xe_mmio_gem_create().
+ *
+ * See: "Exposing MMIO regions to userspace"
+ */
+void xe_mmio_gem_destroy(struct xe_mmio_gem *gem)
+{
+	xe_mmio_gem_free(&gem->base);
+}
+
+static int xe_mmio_gem_mmap(struct drm_gem_object *base, struct vm_area_struct *vma)
+{
+	if (vma->vm_end - vma->vm_start != base->size)
+		return -EINVAL;
+
+	if ((vma->vm_flags & VM_SHARED) == 0)
+		return -EINVAL;
+
+	/* Set vm_pgoff (used as a fake buffer offset by DRM) to 0 */
+	vma->vm_pgoff = 0;
+	vma->vm_page_prot = pgprot_noncached(vm_get_page_prot(vma->vm_flags));
+	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP |
+		     VM_DONTCOPY | VM_NORESERVE);
+
+	/* Defer actual mapping to the fault handler. */
+	return 0;
+}
+
+static void xe_mmio_gem_release_dummy_page(struct drm_device *dev, void *res)
+{
+	__free_page((struct page *)res);
+}
+
+static vm_fault_t xe_mmio_gem_vm_fault_dummy_page(struct vm_area_struct *vma)
+{
+	struct drm_gem_object *base = vma->vm_private_data;
+	struct drm_device *dev = base->dev;
+	vm_fault_t ret = VM_FAULT_NOPAGE;
+	struct page *page;
+	unsigned long pfn;
+	unsigned long i;
+
+	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+	if (!page)
+		return VM_FAULT_OOM;
+
+	if (drmm_add_action_or_reset(dev, xe_mmio_gem_release_dummy_page, page))
+		return VM_FAULT_OOM;
+
+	pfn = page_to_pfn(page);
+
+	/* Map the entire VMA to the same dummy page */
+	for (i = 0; i < base->size; i += PAGE_SIZE) {
+		unsigned long addr = vma->vm_start + i;
+
+		ret = vmf_insert_pfn(vma, addr, pfn);
+		if (ret & VM_FAULT_ERROR)
+			break;
+	}
+
+	return ret;
+}
+
+static vm_fault_t xe_mmio_gem_vm_fault(struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct drm_gem_object *base = vma->vm_private_data;
+	struct xe_mmio_gem *obj = to_xe_mmio_gem(base);
+	struct drm_device *dev = base->dev;
+	vm_fault_t ret = VM_FAULT_NOPAGE;
+	unsigned long i;
+	int idx;
+
+	if (!drm_dev_enter(dev, &idx)) {
+		/*
+		 * Provide a dummy page to avoid SIGBUS for events such as hot-unplug.
+		 * This gives the userspace the option to recover instead of crashing.
+		 * It is assumed the userspace will receive the notification via some
+		 * other channel (e.g. drm uevent).
+		 */
+		return xe_mmio_gem_vm_fault_dummy_page(vma);
+	}
+
+	for (i = 0; i < base->size; i += PAGE_SIZE) {
+		unsigned long addr = vma->vm_start + i;
+		unsigned long phys_addr = obj->phys_addr + i;
+
+		ret = vmf_insert_pfn(vma, addr, PHYS_PFN(phys_addr));
+		if (ret & VM_FAULT_ERROR)
+			break;
+	}
+
+	drm_dev_exit(idx);
+	return ret;
+}

drivers/gpu/drm/xe/xe_mmio_gem.h

···
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright © 2025 Intel Corporation
+ */
+
+#ifndef _XE_MMIO_GEM_H_
+#define _XE_MMIO_GEM_H_
+
+#include <linux/types.h>
+
+struct drm_file;
+struct xe_device;
+struct xe_mmio_gem;
+
+struct xe_mmio_gem *xe_mmio_gem_create(struct xe_device *xe, struct drm_file *file,
+				       phys_addr_t phys_addr, size_t size);
+u64 xe_mmio_gem_mmap_offset(struct xe_mmio_gem *gem);
+void xe_mmio_gem_destroy(struct xe_mmio_gem *gem);
+
+#endif /* _XE_MMIO_GEM_H_ */