/* Copyright (C) 2004  David Decotigny
    Copyright (C) 1999  Free Software Foundation, Inc.
 
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.
    
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    
    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
    USA. 
 */
 #include "segment.h"
 
 #include "gdt.h"
 
 
 /**
  * The sructure of a segment descriptor.
  *
  * @see Intel x86 doc, Vol 3, section 3.4.3, figure 3-8. For segment
  * types, see section 3.5
  */
 struct x86_segment_descriptor
 {
   /* Lowest dword */
   sos_ui16_t limit_15_0;            /* Segment limit, bits 15..0 */
   sos_ui16_t base_paged_addr_15_0;  /* Base address, bits 15..0 */
 
   /* Highest dword */
   sos_ui8_t  base_paged_addr_23_16; /* Base address bits 23..16 */
   sos_ui8_t  segment_type:4;        /* Section 3.4.3.1 (code/data)
                                        and 3.5 (system) of Intel x86 vol 3 */
   sos_ui8_t  descriptor_type:1;     /* 0=system, 1=Code/Data */
   sos_ui8_t  dpl:2;
   sos_ui8_t  present:1;
 
   sos_ui8_t  limit_19_16:4;         /* Segment limit, bits 19..16 */
   sos_ui8_t  custom:1;
   sos_ui8_t  zero:1;
   sos_ui8_t  op_size:1;             /* 0=16bits instructions, 1=32bits */
   sos_ui8_t  granularity:1;         /* 0=limit in bytes, 1=limit in pages */
   
   sos_ui8_t  base_paged_addr_31_24; /* Base address bits 31..24 */
 } __attribute__ ((packed, aligned (8)));
 
 
 /**
  * The GDT register, which stores the address and size of the
  * GDT.
  *
  * @see Intel x86 doc vol 3, section 2.4, figure 2-4; and section
  * 3.5.1
  */
 struct x86_gdt_register {
   /* The maximum GDT offset allowed to access an entry in the GDT */
   sos_ui16_t  limit;
 
   /* This is not exactly a "virtual" address, ie an adddress such as
      those of instructions and data; this is a "linear" address, ie an
      address in the paged memory. However, in SOS we configure the
      segmented memory as a "flat" space: the 0-4GB segment-based (ie
      "virtual") addresses directly map to the 0-4GB paged memory (ie
      "linear"), so that the "linear" addresses are numerically equal
      to the "virtual" addresses: this base_addr will thus be the same
      as the address of the gdt array */
   sos_ui32_t base_addr;
 } __attribute__((packed, aligned(8)));
 
 
 /**
  * Helper macro that builds a Segment descriptor for the virtual
  * 0..4GB addresses to be mapped to the linear 0..4GB linear
  * addresses.
  */
 #define BUILD_GDTE(descr_privilege_level,is_code)               \
   ((struct x86_segment_descriptor) {                            \
       .limit_15_0=            0xffff,                           \
       .base_paged_addr_15_0=  0,                                \
       .base_paged_addr_23_16= 0,                                \
       .segment_type=          ((is_code)?0xb:0x3),              \
              /* With descriptor_type (below) = 1 (code/data),   \
               * see Figure 3-1 of section 3.4.3.1 in Intel      \
               * x86 vol 3:                                      \
               *   - Code (bit 3 = 1):                           \
               *     bit 0: 1=Accessed                           \
               *     bit 1: 1=Readable                           \
               *     bit 2: 0=Non-Conforming                     \
               *   - Data (bit 3 = 0):                           \
               *     bit 0: 1=Accessed                           \
               *     bit 1: 1=Writable                           \
               *     bit 2: 0=Expand up (stack-related)          \
               * For Conforming/non conforming segments, see     \
               * Intel x86 Vol 3 section 4.8.1.1                 \
               */                                                \
       .descriptor_type=       1,  /* 1=Code/Data */             \
       .dpl=                   ((descr_privilege_level) & 0x3),  \
       .present=               1,                                \
       .limit_19_16=           0xf,                              \
       .custom=                0,                                \
       .op_size=               1,  /* 32 bits instr/data */      \
       .granularity=           1   /* limit is in 4kB Pages */   \
   })
 
 
 /** The actual GDT */
 static struct x86_segment_descriptor gdt[] = {
   [SOS_SEG_NULL]  = (struct x86_segment_descriptor){ 0, },
   [SOS_SEG_KCODE] = BUILD_GDTE(0, 1),
   [SOS_SEG_KDATA] = BUILD_GDTE(0, 0),
 };
 
 sos_ret_t sos_gdt_setup(void)
 {
   struct x86_gdt_register gdtr;
 
   /* Address of the GDT */
   gdtr.base_addr = (sos_ui32_t) gdt;
 
   /* The limit is the maximum offset in bytes from the base address of
      the GDT */
   gdtr.limit     = sizeof(gdt) - 1;
 
   /* Commit the GDT into the CPU, and update the segment
      registers. The CS register may only be updated with a long jump
      to an absolute address in the given segment (see Intel x86 doc
      vol 3, section 4.8.1). */
   asm volatile ("lgdt %0          \n\
                  ljmp %1,$1f      \n\
                  1:               \n\
                  movw %2, %%ax    \n\
                  movw %%ax,  %%ss \n\
                  movw %%ax,  %%ds \n\
                  movw %%ax,  %%es \n\
                  movw %%ax,  %%fs \n\
                  movw %%ax,  %%gs"
                 :
                 :"m"(gdtr),
                  "i"(SOS_BUILD_SEGMENT_REG_VALUE(0, FALSE, SOS_SEG_KCODE)),
                  "i"(SOS_BUILD_SEGMENT_REG_VALUE(0, FALSE, SOS_SEG_KDATA))
                 :"memory","eax");
 
   return SOS_OK;
 }