drm/amdgpu: Reduce VA_RESERVED_BOTTOM to 64KB

The reservation is there to catch NULL pointer dereferences from the
GPU. Reduce the size to 64KB to make sure that shared virtual address
programming models can map all CPU-accessible virtual addresses for GPU
access. This is also the default for CPU virtual address mappings as
seen in /proc/sys/vm/mmap_min_addr.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h

@@ -135,10 +135,10 @@ struct amdgpu_mem_stats;
 #define AMDGPU_IS_MMHUB0(x) ((x) >= AMDGPU_MMHUB0_START && (x) < AMDGPU_MMHUB1_START)
 #define AMDGPU_IS_MMHUB1(x) ((x) >= AMDGPU_MMHUB1_START && (x) < AMDGPU_MAX_VMHUBS)
 
-/* Reserve 2MB at top/bottom of address space for kernel use */
+/* Reserve space at top/bottom of address space for kernel use */
 #define AMDGPU_VA_RESERVED_CSA_SIZE		(2ULL << 20)
 #define AMDGPU_VA_RESERVED_SEQ64_SIZE		(2ULL << 20)
-#define AMDGPU_VA_RESERVED_BOTTOM		(2ULL << 20)
+#define AMDGPU_VA_RESERVED_BOTTOM		(1ULL << 16)
 #define AMDGPU_VA_RESERVED_TOP			(AMDGPU_VA_RESERVED_SEQ64_SIZE + \
 						 AMDGPU_VA_RESERVED_CSA_SIZE)