XEN: nopaging_gdt_descr

[http://hi.baidu.com/realvm]51Testing软件测试网R\B5iL

在 ./arch/x86/boot/x86_32.S 中, nopaging_gdt_descr 的内容是这样的：51Testing软件测试网(Vyh-RK)l4GB

5j N6e#S;U3i+d0d+U;hJ0     50 __start:51Testing软件测试网bq^1A;[t(z
     51         /* Set up a few descrīptors: on entry only CS is guaranteed good. */51Testing软件测试网%l%b7F` d{^LU
     52         lgdt    %cs:nopaging_gdt_descr-__PAGE_OFFSET
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ZC,D](pfox0    215 nopaging_gdt_descr:51Testing软件测试网!v|/Y^ o
    216         .word   LAST_RESERVED_GDT_BYTE51Testing软件测试网*^)O$r+e5v{:May
    217         .long   gdt_table - FIRST_RESERVED_GDT_BYTE - __PAGE_OFFSET
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P-n mvW*A0    21851Testing软件测试网0`4gV(Os6V1b|j6^
    219         .align PAGE_SIZE, 0
@wZ;A UV}["|5~/?0    220 /* NB. Rings != 0 get access up to MACH2PHYS_VIRT_END. This allows access to */
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E0    221 /*     the machine->physical mapping table. Ring 0 can access all memory.    */
^t{/eScnd+N0    222 #define GUEST_DESC(d)                                                   \
:_8fy8QJ1tjV0z0    223         .long ((MACH2PHYS_VIRT_END - 1) >> 12) & 0xffff,                \51Testing软件测试网]i3S!Q#b0e'l3t3M
    224               ((MACH2PHYS_VIRT_END - 1) >> 12) & (0xf << 16) | (d)
1JA ft^X?W0pw0    225 ENTRY(gdt_table)51Testing软件测试网$txc1WL0p1`+K
    226         .quad 0x0000000000000000     /* unused */51Testing软件测试网(kh]"?!F
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    227         .quad 0x00cf9a000000ffff     /* 0xe008 ring 0 4.00GB code at 0x0 */
6a$ZW7T,LJK4Lj0    228         .quad 0x00cf92000000ffff     /* 0xe010 ring 0 4.00GB data at 0x0 */
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dV)J*e({-M+x0lgdt    %cs:nopaging_gdt_descr-__PAGE_OFFSET51Testing软件测试网3r8NtllM*j:Z|&@
没有什么，-__PAGE_OFFSET 也解释过了，关键是nopaging_gdt_descr的内容：
&j2U9r.o*t3Rd0    216         .word   LAST_RESERVED_GDT_BYTE51Testing软件测试网tCt.T5bG(x
    217         .long   gdt_table - FIRST_RESERVED_GDT_BYTE - __PAGE_OFFSET51Testing软件测试网&A J+d2u&O
C-x#@
limit 的值是 LAST_RESERVED_GDT_BYTE
R?AJk uM"I(jO0base 的值是 gdt_table - FIRST_RESERVED_GDT_BYTE51Testing软件测试网,m1fAv
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#define FIRST_RESERVED_GDT_PAGE 1451Testing软件测试网}
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#define FIRST_RESERVED_GDT_BYTE (FIRST_RESERVED_GDT_PAGE * 4096)51Testing软件测试网wJdlnEN/p
#define LAST_RESERVED_GDT_BYTE \
\C,b4xf c'dc nRmn(S+A:a0    (FIRST_RESERVED_GDT_BYTE + NR_RESERVED_GDT_BYTES - 1)
#yc%r}c3TM0
{i9^5j!D0从上面的定义可以看出，gdt 的基地址位于gdt_table前，空出了 14 * 4096 /8 ＝ 7168 项
7L$s*J7S JS\D0limit 值包含了gdt_table后4096/8＝512 项51Testing软件测试网T5o CC2\.PT8F
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51Testing软件测试网9}u)a,_/Pma9|RL6H
接下来，51Testing软件测试网E5w"Hv\G*A
     53         mov     $(__HYPERVISOR_DS),%ecx51Testing软件测试网z
HgS@ q"\[ig
     54         mov     %ecx,%ds
A5B|p$y0     55         mov     %ecx,%es51Testing软件测试网y[6`9t:J-vY:X/A
     56         mov     %ecx,%fs51Testing软件测试网so6F s2V+h"J#W7J
     57         mov     %ecx,%gs51Testing软件测试网pRNkA
     58         ljmp    $(__HYPERVISOR_CS),$(1f)-__PAGE_OFFSET
:e*[Rd7_b0__HYPERVISOR_DS， __HYPERVISOR_CS 的值分别是0xe010， 0xe008换成容易看样子：51Testing软件测试网vCP&@:u'o m |
$(__HYPERVISOR_DS) = 0x0000e010 ＝ 57360 ＝ 7170 * 851Testing软件测试网2u] I0t-S
$(__HYPERVISOR_CS) = 0x0000e008 ＝ 57352 ＝ 7169 * 8
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5gA|H"m^0所以，ＣＳ，ＤＳ选择子的内容就是gdt_table的第一，二项的内容51Testing软件测试网o![+vH bL4iML