drm/i915: Reserve some kernel space per vm

Reserve one page in each vm for kernel space to use for things
such as workarounds.

v2: use real memory, do not decrease vm.total
v4: reserve only one page and explain flag
v5: remove allocated object on ppgtt cleanup
v6: decrease vm->total by reservation size
Suggested-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231026-wabb-v6-1-4aa7d55d0a8a@intel.com

drivers/gpu/drm/i915/gt/gen8_ppgtt.c

@@ -5,6 +5,7 @@
 
 #include <linux/log2.h>
 
+#include "gem/i915_gem_internal.h"
 #include "gem/i915_gem_lmem.h"
 
 #include "gen8_ppgtt.h"
@@ -222,6 +223,9 @@ static void gen8_ppgtt_cleanup(struct i915_address_space *vm)
 {
 	struct i915_ppgtt *ppgtt = i915_vm_to_ppgtt(vm);
 
+	if (vm->rsvd.obj)
+		i915_gem_object_put(vm->rsvd.obj);
+
 	if (intel_vgpu_active(vm->i915))
 		gen8_ppgtt_notify_vgt(ppgtt, false);
 
@@ -950,6 +954,41 @@ gen8_alloc_top_pd(struct i915_address_space *vm)
 	return ERR_PTR(err);
 }
 
+static int gen8_init_rsvd(struct i915_address_space *vm)
+{
+	struct drm_i915_private *i915 = vm->i915;
+	struct drm_i915_gem_object *obj;
+	struct i915_vma *vma;
+	int ret;
+
+	/* The memory will be used only by GPU. */
+	obj = i915_gem_object_create_lmem(i915, PAGE_SIZE,
+					  I915_BO_ALLOC_VOLATILE |
+					  I915_BO_ALLOC_GPU_ONLY);
+	if (IS_ERR(obj))
+		obj = i915_gem_object_create_internal(i915, PAGE_SIZE);
+	if (IS_ERR(obj))
+		return PTR_ERR(obj);
+
+	vma = i915_vma_instance(obj, vm, NULL);
+	if (IS_ERR(vma)) {
+		ret = PTR_ERR(vma);
+		goto unref;
+	}
+
+	ret = i915_vma_pin(vma, 0, 0, PIN_USER | PIN_HIGH);
+	if (ret)
+		goto unref;
+
+	vm->rsvd.vma = i915_vma_make_unshrinkable(vma);
+	vm->rsvd.obj = obj;
+	vm->total -= vma->node.size;
+	return 0;
+unref:
+	i915_gem_object_put(obj);
+	return ret;
+}
+
 /*
  * GEN8 legacy ppgtt programming is accomplished through a max 4 PDP registers
  * with a net effect resembling a 2-level page table in normal x86 terms. Each
@@ -1031,6 +1070,10 @@ struct i915_ppgtt *gen8_ppgtt_create(struct intel_gt *gt,
 	if (intel_vgpu_active(gt->i915))
 		gen8_ppgtt_notify_vgt(ppgtt, true);
 
+	err = gen8_init_rsvd(&ppgtt->vm);
+	if (err)
+		goto err_put;
+
 	return ppgtt;
 
 err_put:

drivers/gpu/drm/i915/gt/intel_gtt.h

@@ -249,6 +249,10 @@ struct i915_address_space {
 	struct work_struct release_work;
 
 	struct drm_mm mm;
+	struct {
+		struct drm_i915_gem_object *obj;
+		struct i915_vma *vma;
+	} rsvd;
 	struct intel_gt *gt;
 	struct drm_i915_private *i915;
 	struct device *dma;