x86/idt: Add comments about early #PF handling

The difference between 32 and 64 bit vs. early #PF handling is not
documented. Replace the FIXME at idt_setup_early_pf() with proper comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200528145522.807135882@linutronix.de

diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 4b99f7b..5ef82fc 100644 (file)
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -61,7 +61,11 @@ static bool idt_setup_done __initdata;
 static const __initconst struct idt_data early_idts[] = {
        INTG(X86_TRAP_DB,               asm_exc_debug),
        SYSG(X86_TRAP_BP,               asm_exc_int3),
+
 #ifdef CONFIG_X86_32
+       /*
+        * Not possible on 64-bit. See idt_setup_early_pf() for details.
+        */
        INTG(X86_TRAP_PF,               asm_exc_page_fault),
 #endif
 };
@@ -256,8 +260,10 @@ void __init idt_setup_traps(void)
  * cpu_init() is invoked and sets up TSS. The IST variant is installed
  * after that.
  *
- * FIXME: Why is 32bit and 64bit installing the PF handler at different
- * places in the early setup code?
+ * Note, that X86_64 cannot install the real #PF handler in
+ * idt_setup_early_traps() because the memory intialization needs the #PF
+ * handler from the early_idt_handler_array to initialize the early page
+ * tables.
  */
 void __init idt_setup_early_pf(void)
 {