From e9f86e351fda5b3c40192fc3990453613f160779 Mon Sep 17 00:00:00 2001 From: Zachary Amsden Date: Sat, 3 Sep 2005 15:56:45 -0700 Subject: [PATCH] [PATCH] x86: remove redundant TSS clearing 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 Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/i386/power/cpu.c | 1 - 1 file changed, 1 deletion(-) diff --git a/arch/i386/power/cpu.c b/arch/i386/power/cpu.c index c7a6436aa93..7b0b9ad848e 100644 --- a/arch/i386/power/cpu.c +++ b/arch/i386/power/cpu.c @@ -84,7 +84,6 @@ static void fix_processor_context(void) 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 */ -- 2.41.1