[kernel] Use IST for kernel stacks for NMI, #DF, #PF

We started actually running up against the page boundary for kernel
stacks and thus double-faulting on page faults from kernel space. So I
finally added IST stacks. Note that we currently just
increment/decrement the IST entry by a page when we enter the handler to
avoid clobbering on re-entry, but this means:

* these handlers need to be able to operate with only a page of stack
* kernel stacks always have to be >1 pages
* the amount of nesting possible is tied to the kernel stack size.

These seem fine for now, but we should maybe find a way to use something
besides g_kernel_stacks to set up the IST stacks if/when this becomes an
issue.

src/kernel/gdt.h

@@ -17,17 +17,42 @@ void idt_set_entry(uint8_t i, uint64_t addr, uint16_t selector, uint8_t flags);
 /// Set the stack pointer for a given ring in the TSS
 /// \arg ring  Ring to set for (0-2)
 /// \arg rsp   Stack pointer to set
-void tss_set_stack(int ring, uintptr_t rsp);
+void tss_set_stack(unsigned ring, uintptr_t rsp);
 
 /// Get the stack pointer for a given ring in the TSS
 /// \arg ring  Ring to get (0-2)
 /// \returns   Stack pointers for that ring
-uintptr_t tss_get_stack(int ring);
+uintptr_t tss_get_stack(unsigned ring);
+
+/// Set the given IDT entry to use the given IST entry
+/// \arg i     Which IDT entry to set
+/// \arg ist   Which IST entry to set (1-7)
+void idt_set_ist(unsigned i, unsigned ist);
+
+/// Set the stack pointer for a given IST in the TSS
+/// \arg ist   Which IST entry to set (1-7)
+/// \arg rsp   Stack pointer to set
+void tss_set_ist(unsigned ist, uintptr_t rsp);
+
+/// Increment the stack pointer for the given vector,
+/// if it's using an IST entry
+/// \arg i     Which IDT entry to use
+void ist_increment(unsigned i);
+
+/// Decrement the stack pointer for the given vector,
+/// if it's using an IST entry
+/// \arg i     Which IDT entry to use
+void ist_decrement(unsigned i);
+
+/// Get the stack pointer for a given IST in the TSS
+/// \arg ring  Which IST entry to get (1-7)
+/// \returns   Stack pointers for that IST entry
+uintptr_t tss_get_ist(unsigned ist);
 
 /// Dump information about the current GDT to the screen
 /// \arg index  Which entry to print, or -1 for all entries
-void gdt_dump(int index = -1);
+void gdt_dump(unsigned index = -1);
 
 /// Dump information about the current IDT to the screen
 /// \arg index  Which entry to print, or -1 for all entries
-void idt_dump(int index = -1);
+void idt_dump(unsigned index = -1);