arch/x86/core/gdt.c

/*
 * Copyright (c) 2011-2014 Wind River Systems, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * @file
 * @brief Global Descriptor Table support
 *
 * This module contains routines for updating the global descriptor table (GDT)
 * for the IA-32 architecture.
 */

#include <linker-defs.h>
#include <toolchain.h>
#include <sections.h>

#include <kernel_structs.h>
#include <arch/cpu.h>
#include <arch/x86/segmentation.h>

/*
 * The RAM based global descriptor table. It is aligned on an 8 byte boundary
 * as the Intel manuals recommend this for best performance, see
 * Section 3.5.1 of IA architecture SW developer manual, Vol 3.
 *
 * TODO: CPU never looks at the 8-byte zero entry at all. Save a few bytes by
 * stuffing the 6-byte pseudo descriptor there.
 */
static struct segment_descriptor _gdt_entries[] __aligned(8) = {
	DT_ZERO_ENTRY,
	DT_CODE_SEG_ENTRY(0, 0xFFFFF, DT_GRAN_PAGE, 0, DT_READABLE,
			  DT_NONCONFORM),
	DT_DATA_SEG_ENTRY(0, 0xFFFFF, DT_GRAN_PAGE, 0, DT_WRITABLE,
			  DT_EXPAND_UP)
};

struct pseudo_descriptor _gdt = DT_INIT(_gdt_entries);