Re: [PATCH] 5/8 Move descriptor table management into the sub-arch layer

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Chris Wright wrote:

* Zachary Amsden ([email protected]) wrote:
This change encapsulates descriptor and task register management.

These will need some merging together, will take a stab tomorrow.


--- linux-2.6.12-xen0-arch.orig/include/asm-i386/desc.h
+++ linux-2.6.12-xen0-arch/include/asm-i386/desc.h
@@ -14,9 +14,6 @@

static inline void __set_tss_desc(unsigned int cpu, unsigned int entry, void *addr)
{
-	_set_tssldt_desc(&per_cpu(cpu_gdt_table, cpu)[entry], (int)addr,
+	_set_tssldt_desc(&get_cpu_gdt_table(cpu)[entry], (int)addr,

What is Xen doing for the GDT on SMP? Does Xen have 16 pages of GDT per CPU?

+++ linux-2.6.12-xen0-arch/include/asm-i386/mach-default/mach_desc.h
@@ -0,0 +1,57 @@
+#ifndef __ASM_MACH_DESC_H
+#define __ASM_MACH_DESC_H
+
+extern struct desc_struct cpu_gdt_table[GDT_ENTRIES];
+DECLARE_PER_CPU(struct desc_struct, cpu_gdt_table[GDT_ENTRIES]);
+#define get_cpu_gdt_table(_cpu) per_cpu(cpu_gdt_table, cpu)
+
+#define _set_tssldt_desc(n,addr,limit,type) \
+__asm__ __volatile__ ("movw %w3,0(%2)\n\t" \
+	"movw %%ax,2(%2)\n\t" \
+	"rorl $16,%%eax\n\t" \
+	"movb %%al,4(%2)\n\t" \
+	"movb %4,5(%2)\n\t" \
+	"movb $0,6(%2)\n\t" \
+	"movb %%ah,7(%2)\n\t" \
+	"rorl $16,%%eax" \
+	: "=m"(*(n)) : "a" (addr), "r"(n), "ir"(limit), "i"(type))

This actually doesn't need to move into sub-arch. You can redefine the call sites (set_ldt_desc / set_tss_desc) to operate on stack (implicit register) values instead and then notify the hypervisor about GDT updates. Course, which way is cleaner looks still TBD.

+static inline void clear_LDT(void)
+{
+	int cpu = get_cpu();
+
+	set_ldt_desc(cpu, &default_ldt[0], 5);
+	load_LDT_desc();
+	put_cpu();
+}
+
+/*
+ * load one particular LDT into the current CPU
+ */
+static inline void load_LDT_nolock(mm_context_t *pc, int cpu)
+{
+	void *segments = pc->ldt;
+	int count = pc->size;
+
+	if (likely(!count)) {
+		segments = &default_ldt[0];
+		count = 5;
+	}
+		
+	set_ldt_desc(cpu, segments, count);
+	load_LDT_desc();
+}
+
+#endif

These two don't actually need to move into sub-arch ; they can call functions that have already moved.

So far looks like we are pretty much on the same page, with mostly cosmetic differences.

Zach
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