x86: make GDT per-CPU

The major issue with supporting a significantly larger number of
physical CPUs appears to be the use of per-CPU GDT entries - at
present, x86-64 could support only up to 126 CPUs (with code changes
to also use the top-most GDT page, that would be 254). Instead of
trying to go with incremental steps here, by converting the GDT itself
to be per-CPU, limitations in that respect go away entirely.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index a4f83a1d63..34f4ecf59c 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -213,15 +213,14 @@ void show_page_walk(unsigned long addr)
 asmlinkage void double_fault(void);
 asmlinkage void do_double_fault(struct cpu_user_regs *regs)
 {
-    unsigned int cpu, tr;
-
-    asm volatile ( "str %0" : "=r" (tr) );
-    cpu = ((tr >> 3) - __FIRST_TSS_ENTRY) >> 2;
+    unsigned int cpu;
 
     watchdog_disable();
 
     console_force_unlock();
 
+    asm ( "lsll %1, %0" : "=r" (cpu) : "rm" (PER_CPU_GDT_ENTRY << 3) );
+
     /* Find information saved during fault and dump it to the console. */
     printk("*** DOUBLE FAULT ***\n");
     print_xen_info();