[PATCH RFC 19/44] x86/boot: Defer TSS/IST setup until later during boot on the BSP

[Andrew Cooper <andrew.cooper3@citrix.com>]

TSS and IST setings are only required for safety when running userspace code.
Until we start executing dom0, the boot path is perfectly capable of handling
exceptions and interrupts without a loaded TSS.

Deferring the TSS setup is necessary to facilitiate moving the BSP onto a
percpu stack, which in turn requires that during boot, there are no IST
references in the IDT.

Correct TSS and IST settings are re-set up in reinit_bsp_stack(), just before
we complete initialisation.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
 xen/arch/x86/setup.c | 17 ++++++++++++++++-
 xen/arch/x86/traps.c |  3 ---
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index ffa7ea4..5fa70bd 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -625,6 +625,9 @@ static void __init noreturn reinit_bsp_stack(void)
 {
     unsigned long *stack = (void*)(get_stack_bottom() & ~(STACK_SIZE - 1));
 
+    /* Sanity check that IST settings weren't set up before this point. */
+    ASSERT(MASK_EXTR(idt_tables[0][TRAP_nmi].a, 7UL << 32) == 0);
+
     /* Update TSS and ISTs */
     load_system_tables();
 
@@ -692,7 +695,19 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     percpu_init_areas();
 
     init_idt_traps();
-    load_system_tables();
+    {
+        const struct desc_ptr gdtr = {
+            .base = (unsigned long)this_cpu(gdt_table) - FIRST_RESERVED_GDT_BYTE,
+            .limit = LAST_RESERVED_GDT_BYTE,
+        };
+        const struct desc_ptr idtr = {
+            .base = (unsigned long)idt_table,
+            .limit = (IDT_ENTRIES * sizeof(idt_entry_t)) - 1,
+        };
+
+        lgdt(&gdtr);
+        lidt(&idtr);
+    }
 
     smp_prepare_boot_cpu();
     sort_exception_tables();
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index d06ad69..3eab6d3 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1883,9 +1883,6 @@ void __init init_idt_traps(void)
     set_intr_gate(TRAP_machine_check,&machine_check);
     set_intr_gate(TRAP_simd_error,&simd_coprocessor_error);
 
-    /* Specify dedicated interrupt stacks for NMI, #DF, and #MC. */
-    enable_each_ist(idt_table);
-
     /* CPU0 uses the master IDT. */
     idt_tables[0] = idt_table;
 
-- 
2.1.4


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