[PATCH v4] drm/xe: Support for mmap-ing mmio regions

[Lucas De Marchi]

On Mon, Jul 14, 2025 at 03:26:58PM +0300, Ilia Levi wrote:
>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 <ilia.levi at intel.com>

applied to drm-xe-next.

Thanks,
Lucas De Marchi

>---
> drivers/gpu/drm/xe/Makefile      |   1 +
> drivers/gpu/drm/xe/xe_mmio_gem.c | 226 +++++++++++++++++++++++++++++++
> drivers/gpu/drm/xe/xe_mmio_gem.h |  20 +++
> 3 files changed, 247 insertions(+)
> create mode 100644 drivers/gpu/drm/xe/xe_mmio_gem.c
> create mode 100644 drivers/gpu/drm/xe/xe_mmio_gem.h
>
>diff --git a/drivers/gpu/drm/xe/Makefile b/drivers/gpu/drm/xe/Makefile
>index 4bda20f586ed..382aadd4e816 100644
>--- a/drivers/gpu/drm/xe/Makefile
>+++ b/drivers/gpu/drm/xe/Makefile
>@@ -85,6 +85,7 @@ xe-y += xe_bb.o \
> 	xe_lrc.o \
> 	xe_migrate.o \
> 	xe_mmio.o \
>+	xe_mmio_gem.o \
> 	xe_mocs.o \
> 	xe_module.o \
> 	xe_nvm.o \
>diff --git a/drivers/gpu/drm/xe/xe_mmio_gem.c b/drivers/gpu/drm/xe/xe_mmio_gem.c
>new file mode 100644
>index 000000000000..9a97c4387e4f
>--- /dev/null
>+++ b/drivers/gpu/drm/xe/xe_mmio_gem.c
>@@ -0,0 +1,226 @@
>+// 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;
>+}
>diff --git a/drivers/gpu/drm/xe/xe_mmio_gem.h b/drivers/gpu/drm/xe/xe_mmio_gem.h
>new file mode 100644
>index 000000000000..4b76d5586ebb
>--- /dev/null
>+++ b/drivers/gpu/drm/xe/xe_mmio_gem.h
>@@ -0,0 +1,20 @@
>+/* 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_ */
>-- 
>2.43.0
>