[PATCH] i386 descriptor cleanup and question

[Chuck Ebbert]

 There is no need to clear the busy bit when setting up TSS entries
in the GDT -- __set_tss_desc() creates them that way.  This patch
documents that and removes the unnecessary code.  It also documents
the fixed bits in LDT entries.  Tested on uniprocessor only -- I
dumped the GDT and it looks right, with entry 31 having flags 89
while #16 has flags 8b (busy bit set by "ltr" instruction.)

 Why is there only one TSS for doublefault handling on SMP?  Is the
chance of multiple concurrent faults too small to be worth handling?

 arch/i386/kernel/cpu/common.c |    5 ++---
 include/asm-i386/desc.h       |   10 ++++++----
 2 files changed, 8 insertions(+), 7 deletions(-)

diff -u --exclude-from=/home/me/.exclude -r linux-2.5.66-ref/arch/i386/kernel/cpu/common.c linux-2.5.66-uni/arch/i386/kernel/cpu/common.c
--- linux-2.5.66-ref/arch/i386/kernel/cpu/common.c	Sat Mar 29 09:16:21 2003
+++ linux-2.5.66-uni/arch/i386/kernel/cpu/common.c	Sun Apr  6 07:13:50 2003
@ -487,13 +487,12 @
 
 	load_esp0(t, thread->esp0);
 	set_tss_desc(cpu,t);
-	cpu_gdt_table[cpu][GDT_ENTRY_TSS].b &= 0xfffffdff;
 	load_TR_desc();
 	load_LDT(&init_mm.context);
 
-	/* Set up doublefault TSS pointer in the GDT */
+	/* Set up doublefault TSS descriptor in the GDT
+	 */
 	__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS, &doublefault_tss);
-	cpu_gdt_table[cpu][GDT_ENTRY_DOUBLEFAULT_TSS].b &= 0xfffffdff;
 
 	/* Clear %fs and %gs. */
 	asm volatile ("xorl %eax, %eax; movl %eax, %fs; movl %eax, %gs");
diff -u --exclude-from=/home/me/.exclude -r linux-2.5.66-ref/include/asm-i386/desc.h linux-2.5.66-uni/include/asm-i386/desc.h
--- linux-2.5.66-ref/include/asm-i386/desc.h	Tue Mar  4 22:28:52 2003
+++ linux-2.5.66-uni/include/asm-i386/desc.h	Sun Apr  6 07:15:21 2003
@ -44,6 +44,7 @
 
 static inline void __set_tss_desc(unsigned int cpu, unsigned int entry, void *addr)
 {
+	/* type 0x89: present, not-busy TSS, DPL=0 */
 	_set_tssldt_desc(&cpu_gdt_table[cpu][entry], (int)addr, 235, 0x89);
 }
 
@ -57,8 +58,9 @
 #define LDT_entry_a(info) \
 	((((info)->base_addr & 0x0000ffff) << 16) | ((info)->limit & 0x0ffff))
 
-#define LDT_entry_b(info) \
-	(((info)->base_addr & 0xff000000) | \
+/* const 0x7000: DPL = 3, type = code or data */
+#define LDT_entry_b(info) ( \
+	((info)->base_addr & 0xff000000) | \
 	(((info)->base_addr & 0x00ff0000) >> 16) | \
 	((info)->limit & 0xf0000) | \
 	(((info)->read_exec_only ^ 1) << 9) | \
@ -67,9 +69,9 @
 	((info)->seg_32bit << 22) | \
 	((info)->limit_in_pages << 23) | \
 	((info)->useable << 20) | \
-	0x7000)
+	0x7000 )
 
-#define LDT_empty(info) (\
+#define LDT_empty(info) ( \
 	(info)->base_addr	== 0	&& \
 	(info)->limit		== 0	&& \
 	(info)->contents	== 0	&& \

--
 Chuck
 I am not a number!