When reviewing GDT updates, I found the code:
set_tss_desc(cpu,t); /* This just modifies memory; ... */
per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS].b &= 0xfffffdff;
This second line is unnecessary, since set_tss_desc() has already cleared
the busy bit.
Commented disassembly, line 1:
c028b8bd: 8b 0c 86 mov (%esi,%eax,4),%ecx
c028b8c0: 01 cb add %ecx,%ebx
c028b8c2: 8d 0c 39 lea (%ecx,%edi,1),%ecx
=> %ecx = per_cpu(cpu_gdt_table, cpu)
c028b8c5: 8d 91 80 00 00 00 lea 0x80(%ecx),%edx
=> %edx = &per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS]
c028b8cb: 66 c7 42 00 73 20 movw $0x2073,0x0(%edx)
c028b8d1: 66 89 5a 02 mov %bx,0x2(%edx)
c028b8d5: c1 cb 10 ror $0x10,%ebx
c028b8d8: 88 5a 04 mov %bl,0x4(%edx)
c028b8db: c6 42 05 89 movb $0x89,0x5(%edx)
=> ((char *)%edx)[5] = 0x89
(equivalent) ((char *)per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS])[5] = 0x89
c028b8df: c6 42 06 00 movb $0x0,0x6(%edx)
c028b8e3: 88 7a 07 mov %bh,0x7(%edx)
c028b8e6: c1 cb 10 ror $0x10,%ebx
=> other bits
Commented disassembly, line 2:
c028b8e9: 8b 14 86 mov (%esi,%eax,4),%edx
c028b8ec: 8d 04 3a lea (%edx,%edi,1),%eax
=> %eax = per_cpu(cpu_gdt_table, cpu)
c028b8ef: 81 a0 84 00 00 00 ff andl $0xfffffdff,0x84(%eax)
=> per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS].b &= 0xfffffdff;
(equivalent) ((char *)per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS])[5] &= 0xfd
Note that (0x89 & ~0xfd) == 0; i.e, set_tss_desc(cpu,t) has already stored the
type field in the GDT with the busy bit clear.
Eliminating redundant and obscure code is always a good thing; in fact, I
pointed out this same optimization many moons ago in arch/i386/setup.c, back
when it used to be called that.
Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
Index: linux-2.6.13/arch/i386/power/cpu.c
===================================================================
--- linux-2.6.13.orig/arch/i386/power/cpu.c 2005-08-08 13:50:15.000000000 -0700
+++ linux-2.6.13/arch/i386/power/cpu.c 2005-08-08 14:54:28.000000000 -0700
@@ -84,7 +84,6 @@
struct tss_struct * t = &per_cpu(init_tss, cpu);
set_tss_desc(cpu,t); /* This just modifies memory; should not be necessary. But... This is necessary, because 386 hardware has concept of busy TSS or some similar stupidity. */
- per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_TSS].b &= 0xfffffdff;
load_TR_desc(); /* This does ltr */
load_LDT(¤t->active_mm->context); /* This does lldt */
